Hi Chad,
What is the average size...Well, there sort of isn't one, not exactly.
Your pouch is built out of a certain length of intestine...in the beginning, it is rather small but as time goes by and you can 'hold it' longer you 'grow' your pouch.
The longer you can hold it, the larger it can become (within certain limits)...most often, it holds about 1litre (1 quart) of content plus gas +/-.
If you are larger than the average bear (sorry, Yoggi bear humor) then you will 'grow' your pouch accordingly. According to how much you eat, how often you empty etc.
Whenever I am 'pouch-sick' and have to go off of solids for an extended period of time or have had surgery, my pouch shrinks to about 300ml or just about 1.1 pints.
It takes me about 4-6 weeks to re-stretch it a bit. Up to 6 months for it to reach its size of maturity.
Evidently, you can, within reason, control the size of it by either eating less or emptying more often.
I understand the problem of having a large appetite but maybe you could eat smaller meals a little more often.
Sharon