1. Rabbenu Yonah (Brachot 30a D"H Amar) writes that the simple explanation of Rav Nachman is that if one has the amount that others have as a normal meal one should make Birkat HaMazon even if didn't establish it as a meal. The Tur and S"A 168:6 codifies this as halacha.
2. Rosh (Brachot 6:30) quotes the Rambam who says that if one establishes a meal but doesn't eat the amount that others have as a meal one shouldn't bench, however, the Raavad argues that one must bench. The Rabbenu Yonah (Brachot 30a D"H Amar), Tur, and Shulchan Aruch 168:6 agree with the Rambam. [The Pri Megadim (A"A 168:13) points out that this isn't readily understood from the Rambam in Mishneh Torah (Brachot 3:9).]
How do we define the shiur of a meal?
1. The Chida- The volume of 3 or 4 eggs, which is based off the size of Eruv Techumin
2. The Gra- a meal like breakfast or dinner
3. The Shulchan Aruch HaRav- half the shiur of challah, which is half an Omer, based off the size of the Man which would fall in desert.
3. Sephardic pesak
Sephardim hold definitively like the Chida. Thus, it's preferable to eat less than 3 eggs and make Mezonot or more than 4 eggs and make HaMotzei. 
4. Ashkenazic pesak
Mishna Brurah 168:24 seems to side with the Gra but writes that one should be concerned for the opinion of the Chida. Rav Moshe Feinstein writes that primarily the halacha follows the Gra, however, we are also concerned for the opinion of the Chida and not concerned for the opinion of the Shulchan Aruch HaRav which is more than anyone can eat.
Therefore, to avoid dispute one should eat less than 4 KeBaytzim and make Mezonot or eat more than the average meal and make HaMotzei. After the fact, if one did eat between these two amounts then if one isn't full one should make Al HaMichya and if one is full one should make Birkat HaMazon.
According to Ashkenazim, what does the "a meal like breakfast or dinner" mean?
Rav Vosner assumes that the amount of the Gra is much larger than 4 eggs. However, others argue that nowadays perhaps the amount of "a meal like breakfast or dinner" actually is 4 eggs. Also, Rav Moshe Feinstein holds the shiur varies from country to country, as well as from different categories of people like old and young.
Combining Other Foods
The Magen Avraham 168:13 has a chidush that we do include "other foods". The logic is that a typical bread includes other filling foods. Sephardim generally don't hold like the Magan Avraham, while Ashkenazim do hold like the Magen Avraham.
Some say that the other foods eaten in the meal are counted towards the total only if one already ate 4 Kebaytzim of Pas HaBah Bekisnin besides the other foods, while others say that even if one only has one Kezayit of Pas HaBah Bekisnin together with the amount eaten in a meal of other meal foods one should make HaMotzei. According to this second opinion if a person at a wedding is interested in avoiding washing and substitutes crackers for bread then since the crackers and other foods amount to a Shiur Sueda one would have to wash and make HaMotzei on the crackers.
6. When in the meal is the Pas HaBah Bekisnin eaten?
Rav Sheinburg held that Pas HaBah Bekisnin doesn't combine with the other foods if the other foods don't accompany the Pas HaBah Bekisnin and are eaten separately. On the other hand, Rav Moshe Feinstein held that even if the other foods aren't with the Pas HaBah Bekisnin nonetheless they combine because one did become full from everything together.
 Birkei Yosef 168:4, Eliyah Rabba 168:8 quoting the Or Chadash, Kaf HaChaim 168:45
 Chaye Adam 54:4 quoting the Gra, Shaar HaTzion 168:19 quoting the Magan Avraham
 S"A HaRav 168:8, Aruch HaShulchan 168:16
 Yalkut Yosef 168:8 (vol 3, pg 129), Kaf HaChaim 168:45, Ben Ish Chai (Pinchas 19)
 Sh"t Igrot Moshe OC 3:32
 Halachos of Brachos (chap 27, pg 489, note 26-7) quoting Rav Shlomo Zalman, See similar ruling in Vezot HaBracha (chap 4, pg 31) quoting Rav Elyashiv and Rav Shienburg
 Sh"t Shevet HaLevi 7:25
 See Halachos of Brachos (chap 27, pg 487, note 23) quoting Rav Elyashiv and Rav Shlomo Zalman
 Biur Halacha 168:6 D"H Af Al Pi, Sh"t Igrot Moshe OC 3:33, Halachos of Brachos (chap 27, pg 487-8, note t and 24.1)
 Birkay Yosef 168:6, Kaf HaChaim 168:47
 Mishna Brurah 168:24
 Vezot HaBracha (chap 4, pg 32) quoting Rav Vosner and Rav Shlomo Zalman
 Vezot HaBracha (chap 4, pg 32) quoting Rav Sheinburg, inferring it from Rav Moshe, and quoting Rav Elyashiv who was concerned for this opinion
 Igrot Moshe 3:32, Halachos of Brachos (chap 27, pg 491)
 Vezot HaBracha (chap 4, pg 32) quoting Rav Sheinburg and Rav Shlomo Zalman who wasn't sure about this idea
 Vezot HaBracha (chap 4, pg 32) quoting Rav Moshe and Rav Elyashiv