You must eat a second meal on Shabbat (or Jewish festival) day with at least 1.9 fl. oz. (56 ml, 1/4 cup) of bread--even if you already said ha'motzi but ate less than 1.9 fl. oz. of bread at kiddush.  

To fulfill the two requirements for Jewish festival evening kiddush:

1. Make Kiddush
   Say, or hear, the three Jewish festival evening kiddush blessings/segments:
   1. Borei pri ha'gafen (if on wine or grape juice—preferred option), OR
      Ha'motzi (on two challot if you have no wine or grape juice, since no chamar medina is allowed for Jewish festival evening kiddush; see Jewish Festivals: Kiddush: Challot for Kiddush) AND
   2. Asher bachar banu mi kol am.... mekadeish Yisrael v'hazmanim, AND
   3. She'hecheyanu on all Jewish festival nights, except the last two nights of Passover.  So say she'hecheyanu on:
      • Both nights of Rosh Hashana (in or outside of Eretz Yisrael),
      • First two nights of Passover (1 night in Eretz Yisrael),
      • Both nights of Shavuot (1 night in Eretz Yisrael),
      • First two nights of Sukkot (1 night in Eretz Yisrael), and
      • Nights of Shmini Atzeret and Simchat Torah (same day in Eretz Yisrael).
      Note There is never any requirement on an indvidual to drink kiddush wine (except at the Passover seder), but the kiddush wine must be drunk by one or more persons.
      Note A woman who had said she'hecheyanu when she lit the Jewish festival candles does NOT say she'hecheyanu again if she makes kiddush for herself, even when making kiddush at the Passover seder.
      Note There is no need to eat a new fruit after saying she'hecheyanu on the second night of  Shavuot, Passover, or Sukkot. This is not comparable to Rosh Hashana, since the second festival night was instituted due to uncertainty of the actual date of the holiday, while Rosh Hashana is considered to be one single, long day.
2. Establish a halachic “meal” (kovei'a se'uda).
   For how to establish a halachic meal, see Introduction to Jewish Festival “Eating a Meal” Requirements.

Less than 1/48th missing is still considered a whole loaf. So if you only have two challot (or other loaves of bread) for Shabbat, you might be able to use one loaf twice, as follows:

• Wash your hands,
• Say ha'motzi,
• Cut off a piece that is less than 1/48th of the loaf, and
• Eat it.

REASON You may consider the remainder of that loaf as still being a full loaf and you may re-use it for your Shabbat morning meal.
NOTE If you have pieces of bread or other mezonot, you may:

• Cut off less than 1/48th of the loaf,
• Eat the additional pieces of bread to make a total of at least 1.9 fl. oz. (56ml), and then
• Re-use the same loaf for Shabbat morning.

Less than 1/48th missing is still considered a whole loaf. So if you only have two challot (or other loaves of bread) for a Jewish festival, you might be able to use one loaf twice:

• Wash your hands,
• Say ha'motzi,
• Cut off a piece that is less than 1/48th of the loaf, and
• Eat it.

We are required to eat two meals on Jewish festivals, each preceded by kiddush, one in the evening and one in the morning. For both meals for each Jewish festival day (and all three meals on Shabbat), say ha'motzi over two complete loaves of bread, each of which is at least 1.3 fl. oz. in volume. 

Source of Saying Jewish Festival Kiddush
Some kiddushes are commanded by the Torah (d'oraita); the others are from Chazal (d'rabanan), as follows:
Kiddush d'Oraita

• First night of Jewish festivals
• Night of seventh day of Passover
• Night of Shmini Atzeret.

Kiddush d'Rabanan

• First day of Jewish festival
• Second night of Jewish festival
• Second day of Jewish festival
• Seventh day of Passover
• Eighth night of Passover
• Eighth day of Passover
• Shmini Atzeret day
• Night and day of Simchat Torah.

Source of Jewish Festival Kiddush Location
Saying Jewish festival kiddush at the place where you will eat your meal is a rabbinical (d'rabanan) enactment.
Jewish Festival Kiddush-Meal Quantities: Evening

• For evening kiddush for a first-night (d'oraita) Jewish festival, a minimum of 4 fl. oz. (119 ml) of wine must be blessed on and at least half must be drunk.
• For evening kiddush for a second-night (d'rabanan) Jewish festival, a minimum of 3.3 fl. oz. (99 ml) of wine must be blessed on and at least 2.0 fl. oz. must be drunk.
• For the evening meal, as on the first two Shabbat meals and for all required Jewish festival meals, a minimum of 1.9 fl. oz. (56 ml) of bread must be eaten within four minutes.

Eating Requirements for Jewish Festival Morning Kiddush
There are two separate eating requirements during the daytime. They may be combined (say/hear kiddush and start the main meal right away) but are often done separately (say/hear kiddush and then eat some light foods and beverages; the main meal is eaten later in the day).
Note Since eating and drinking requirements on all morning kiddushes (both Shabbat and Jewish festivals) are d'rabanan, the required beverage amount for morning kiddush is only 3.3 fl. oz. (99 ml) instead of the d'oraita 4 fl. oz. (119 ml) (which is required for kiddushes for Shabbat evening and all first-night Jewish festivals).

• Morning kiddush requires a halachically legal “meal” with these elements:
  • Blessing on a minimum of 3.3 fl. oz. (99 ml) of wine (or other beverage),
  • Someone's drinking at least half the beverage, followed by
  • Eating at least 1.3 fl. oz. (39 ml, or 1/6 cup) of some type of mezonot within four minutes. 
    Note If you drink at least 3.3 fl. oz. (99 ml) of wine within 30 seconds, you do not need to eat mezonot.
    Note You do not need to drink the wine or other kiddush beverage to fulfill “establishing a meal.” You may hear kiddush and then simply eat the required amount of bread or mezonot. This applies to Shabbat or Jewish festivals, evening or morning.
     

 

• The real meal (kovei'a se'uda) requires eating at least 1.9 fl. oz. (56 ml) of bread (or matza during Passover!) within four minutes. It must include enough food to satiate. 
  Note You can simultaneously fulfill the requirement to “establish a meal” and to “eat a meal” by eating one (the same) piece of bread.

The blessing on matza brei is mezonot, as long as the pieces of matza are smaller than 1.3 fl. oz. (39 ml, or 1/6 cup) each. If even one of the pieces is larger than 1.3 fl. oz., wash your hands and say ha'motzi before eating it.

Saying Baruch ha'mavdil bein kodesh l'chol after dark at the end of Rosh Hashana (as for Jewish festivals) does not affect the additions you will then say in birkat ha'mazon.

Women (even if 10 or more) do not make a minyan, only a mezuman.  So do not say the birkat ha'mazon's minyan introduction but simply say the mezuman introduction if:

• 10 or more women ate together, without men present;
• At least two women washed, said ha'motzi, and ate bread; AND
• At least one more woman ate some type of food.

Saying Baruch ha'mavdil bein kodesh l'chol after dark at the end of a Jewish festival or of Rosh Hashana does not affect the additions you will then say in birkat ha'mazon.

If you said a fore-blessing and ate a snack, but then decided to eat bread and a full meal:

• If your meal will not contain any foods that have the same after-blessing as your snack--
  • Say the snack's after-blessing, and then
  • Wash your hands and say ha'motzi.
• If your meal will contain foods with the same fore-blessing as your snack (even the same food as your snack)—
  • Do not say the snack's after-blessing.
  • Wash your hands and say ha'motzi.

Say borei pri ha'adama on vegetable salad.

If you planned to, but did not, eat a full meal:

• Do not say al ha'michya if you did not eat at least 1.3 fl. oz. (39 ml, or 1/6 cup) of foods baked from the Five Grains within four minutes--even if you had washed your hands and said ha'motzi.
• Do say the appropriate bracha achrona for any other foods of which you ate the minimum (1.3 fl. oz.--39 ml, or 1/6 cup) quantity within four minutes, even though you did not say an individual bracha rishona (since you were planning to

Say birkat ha'mazon if you ate a “meal” as defined by halacha—that is, if you ate at least 1.3 fl. oz. (39 ml, or 1/6 cup) of bread made of the Five Grains within four minutes.

Three Shabbat Meals
We are required to eat three Shabbat meals, as a rabbinic (d'rabanan) enactment to enjoy Shabbat (oneg Shabbat). The first Shabbat meal must be at night and the remaining two must be during the day (the third meal must be eaten after halachic midday). Friday night dinner and the first meal on Saturday are preceded by kiddush. For the first two Shabbat meals, say ha'motzi over two complete loaves of bread, each of which is at least 1.3 fl. oz. in volume. For the third meal, the ideal is to use two complete loaves of bread, but the requirement of eating the third meal can also be fulfilled by eating any food other than salt or water.

Source of Saying Shabbat Kiddush

• Saying kiddush on Shabbat night is a commandment from the Torah (d'oraita).
• Saying kiddush on Saturday morning is a rabbinical (d'rabanan) enactment.

Note If you did not say Friday night kiddush, you must say that version of kiddush on Shabbat morning and it is then a requirement from the Torah (d'oraita). Do not begin with va'yechulu; instead, begin with borei pri ha'gafen and say the second blessing of kiddush.

Source of Kiddush Location
Saying kiddush at the place where you will eat your meal is a rabbinical (d'rabanan) enactment.

Shabbat Kiddush-Meal Quantities: Evening

• For evening kiddush, a minimum of 4 fl. oz. (119 ml) of wine must be blessed on and at least half must be drunk.
• For the evening meal, as on Shabbat lunch and all required Jewish festival meals, a minimum of 1.9 fl. oz. of bread must be eaten within four minutes.

Shabbat Kiddush-Meal Quantities: First Meal on Saturday
Shabbat day first meal has two separate eating requirements.  They may be combined (say/hear kiddush and start the main meal right away) but are often done separately (say/hear kiddush and then eat some mezonot; the main meal is eaten later in the day).
Note Since eating and drinking requirements on all morning kiddushes (both Shabbat and Jewish festivals) are d'rabanan, the required beverage amount for morning kiddush is only 3.3 fl. oz. (99 ml) instead of the d'oraita 4 fl. oz. (119 ml), which is required for kiddushes for Shabbat evening.
1) Morning kiddush requires a halachically legal “meal” with these elements:
      a) Blessing on a minimum of 3.3 fl. oz. (99 ml) of wine (or other beverage),
      b) Someone's drinking at least 2 fl. oz. of the beverage, followed by
      c) Eating at least 1.3 fl. oz. (39 ml, or 1/6 cup) of some type of mezonot (or bread) within four minutes.
Note If you drink at least 3.3 fl. oz. (99 ml) of wine within 30 seconds, you do not need to eat mezonot.
Note You do not need to drink the wine or other kiddush beverage to fulfill “establishing a meal.” You may hear kiddush and then simply eat the required amount of bread or mezonot. This applies to Shabbat or Jewish festivals, evening or morning.
The kiddush “meal” does not have to satiate.
Note If you have not fulfilled the requirements for kiddush, you may not eat other foods, such as fruit or fish at a kiddush.

2) The real meal (kovei'a se'uda) of Shabbat lunch requires eating at least 1.9 fl. oz. (56 ml) of bread (or matza during Passover!) within four minutes. It should include enough food to satiate.
Note You can simultaneously fulfill the requirement to “establish a meal” and to “eat a meal” by eating one (the same) piece of bread.
For details on fulfilling the first two meals' requirements, see Shabbat: Kiddush.

Shabbat: Eating a Meal Requirement: Third Meal
For details on fulfilling the third meal's requirements, see Shabbat: Third Meal (Se'uda Shlishit).