"How large did a town have to be to support a general store which sold firearms as well"
Well, from some of the things I've seen over the years, apparently not all that big. Where ever a group of people clustered for some economic reason, it usually drew people who were willing to try to make a buck off those people.
Some of the lumber towns in the Pacific Northwest at the turn of the last century are good examples. Isolated, small, but still had their store and saloon...
What a lot of people also don't realize about these small stores is that they often were much more than simply a store. They were often a restaurant, a post office, an informal bank, the railroad office/telegraph office (if they were located on or near a line) and, in the age of mail order for things that often weren't carried in stores outside the big cities, the order and package acceptance service.
If a town got bigger, sometimes those things would be branched out to other dedicated stores, but you'll still occasionally see places like that in really rural areas -- General store/filling station/post office etc.
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.