I have to agree with Stumpkiller. The 20 gauge is probably the best overall choice. But, any 16, 14, or 12 gauge gun will also be useful. The nice advantage of the larger guns is that you can DOWNLOAD them to produce smaller bore ballistics, so that you can load a 26 gauge equivalent load in that 20, 16, 14 or 12 gauge fowler, and kill quail, or rabbits without any recoil.
Stumpy is correct however, that if you are talking about using a fowler for deer hunting, you are going to be pushing an ever large ball out the barrel, with attendant increases in recoil, as the gauge increases in size. While you can adjust your powder charge somewhat, to reduce recoil, you won't be able to get rid of it all.
I have a DB 12 gauge percussion shotgun, and now have a 20 gauge fowler, in flint. My .50 caliber rifle (flint) will remain my primary deer gun, but that fowler is going to be thoroughly tested with RB to see what kind of accuracy I can get. Right now, I prefer to consider my fowler as a bird gun.