Part also depends (imho) on your loading procedure. I have a different bag for my smoothies, because I have a pretty different procedure for them compared to my rifle.
So, IF you're doing small game, you're going for squirrels or rabbits, and most definitely if you're going for birds, you're going to need a bunch of loads of shot. Some folks like a shot snake, a horn, and a bag holding wads and over-shot cards, plus tools and such. They have two items slung over a shoulder, plus a small bag that way.
Other folks, myself included, like to use paper cartridges a la the military, with cartridges of premeasured loads for the powder, and cartridges of premeasured shot. So the bag needs to hold a bunch of cartridges, plus tools, and the only need is for a small horn to prime with. If using a caplock, you don't even need the horn.
I've found this works very well, as it allows me to put my SxS caplock to its full potential. I can load the right barrel with the wide pattern with #7½ shot hoping to kick up a bird, and the left barrel with #5 or #4 shot and a tighter pattern for squirrels and rabbits. It also works well with my single shot flintlock trade guns, so I use the same procedure for both. In fact once I forgot the bag, but had remembered the ammo can of powder and shot paper cartridges, and the horn, so I just put the cartridges in my coat pockets, and went after squirrels. With the right combination of waistcoat and coat, you might not need a bag, at all.
You'll have to decide what works the best for your gun and decide then what size bag will fit your needs.
LD