How do you intend to use the pouch and for what caliber and kind and of gun, I.E. Rifle or Smoothbore?
If you are only using it for hunting, then a minimalist or "day pouch" is probably all you need because you don't need to carry as much "stuff" in it.
Do you plan on reenacting or doing living history with it and if so, what time period?
If you plan on using it in some primitive matches where once you cross a line, you can not go back and get something you don't normally need, but now need to keep shooting. Then you might want a larger pouch. I have seen folks who had to forfeit such a match at Friendship because they didn't have something in the pouch they needed on a rare occasion.
As others have stated, it really is important you lay out what you are actually going to put in the pouch AND decide how you want them in the pouch. Small loops or pockets inside a pouch can organize some of the gear better and allow you to get by with a smaller pouch.
Gus