Whew! That's like asking how big a car you should drive. Depends on how much you usually carry and your driving style. Some guys lug everything they own, others just a few select items. If you only plan on shooting a few times, you can take a tiny pouch, or none at all if you have a ball-block on the horn strap. If you shoot every 15 minutes for all waking hours you'll need a larger kit.
My hunting pouch:
What's in it:
Tin of Moose Snot, 1 oz bottle of moose milk, "dry" moose milk lubed patching strip in waxed deerskin bag, extra patching, ball bag (25 to 40 balls depending on my plans), a scrap of leather strap for flint leathers or to wrap on the rammer for added grip should it stick. Two flints inside the flap in a small pocket. A tool roll with: mainspring vise, five flints, two vent picks, five vent feathers, a screwdriver/pin pusher/flint nibbler, and a dozen flannel cleaning patches. Five-shot ball block and attached stub starter in a sheath on the back of the bag (starter inside the bag so either keeps the other from getting lost).
Jag & tow in the patchbox. Three-shot ball block, measure and patch knife on the horn strap.
10" Rifleman's knife & 5" skinning knife in belt sheaths, flint & steel tinderbox on belt or inside jacket so it doesn't get lost if bag does. (The tool roll has flints & steel enough to serve in an emergency). Not shown is a leather frizzen stall/emergency powder measure that I toss in the bottom and usually forget.
Anything else I carry in a seperate haversack.
I carry the same bag for all uses, hunting, treking, whatever. 8-1/2" deep and 6-1/2" wide.
.50 cal ball block:
If you can find a 15/32" drill you're all set. I have a 7/16" drill (#7 auger bit) and I have to open the holes a bit with a pen-knife or sandpaper around a dowel.