I have never bred pygmy cories before, but they would be similair to other cories.
A 5 gallon would be fine. I found when I bred my cories they would only eat the eggs, but wouldn't bother with the fry. I used two males and one female.
Well I would go with 4-5 then. The females will be a bit larger and plumper then the males. In most cases when you buy pygmy cories at the LFS they are already full grown. But I would say around 1/2" Someone may correct me on that because I am not totally sure when thet are sexually mature.