Moose is my favorite, but cow is what I have the most of (one moose, two cow, one cow/deer hybrid). I've got an elk side quiver and I think that would be a dandy bag leather, too. Haven't owned an elk bag . . . yet.
I like the little ones, maybe 7 x 7. I have a belt bag that's a tad smaller than that that I prefer for whitetail hunting (as opposed to small game where I need to carry more balls & fixins). Too big a bag and everything you need is on the bottom. A pocket for the jag, and another for spare flints or caps. if it's for a percussion, I like to have a long loop or tube just inside the gussets for my cappers. For a smoothbore it's nice to have three pockets: two shallow ones for over-shot and over-powder cards, and a larger pocket for fiber wads.
Thinner leather is quieter because it forms around all the objects inside and tends to give more when slapped with a branch or scraping through cover.
The bigger the bag, the more unnecessary stuff you'll leave in it. I keep trying to distill my accoutrements down to the absolute bare minimum. Rolls of tic that can be used for patching or cleaning and multi-purpose lube/powder-solvent in a little one ounce bottle or tin.
I carry a seperate haversack with victuals and other necessities that aren't needed for shootin.