These cups are not usually labeled with serving information. They don't, for example, have how many calories are in them because they can be filled with a variety of different sodas (some zero calorie).

FWIW, from my perception, the 128 oz is very rare, 64 oz is fairly common but a small percent of sales, and 44 oz is very common. (edit: these are for convenience stores like 7-11, not restaurants etc where sizes are much smaller)