Well, a .45 will drop pretty much anyone. A .40 will also do the job. A 9mm is smaller but is usually enough for stopping power. Anything smaller than that (.380, .25, .22) might be too small. Then there are the .38's and the .357's. It depends on where you live too. Is it highly populated? Factors like that do make a difference. Also how much you shoot. The bigger the gun, the more expensive the ammo. Hollow points are for carrying and they are expensive. Practice rounds are cheaper and also less powerful b/c they have less grains.
You have freepmail!
"Well, a .45 will drop pretty much anyone."
I'm not going to ask who you love more - your spouse or your kids. But - I would be interested in knowing who you like the least! :-)