Lots of good answers. I think most everyone would agree that a 45, either acp or Colt is the upper limit for self defense in a CCW gun. This limit would be set by controlability of the handgun, concealibility, and also the performance of the ammunition. Other more powerful handguns would also work with custom ammunition designed to expand, and a dedicated shooter willing to master the recoil.
Now for Rural America and a house or truck gun the field is wide open. I have packed my 480 Ruger Super RedHawk out in the yard with a flashlight a few times. Not because I needed the power, but that was what I happened to have handy. It would not be out of the ordinary for me to be packing the 480 in my UTV or truck either. Lots of rural folks will have a 41, and 44 Magnum Single Action revolvers. Is this more gun than you need for self defense? Probably, but you are not going to feel undergunned.
The CCW gun I carry most when running around in town would be a S&W 638 with +P 38's in it. It is Lite, Handy, Accurate, and very Concealible in a Lobo Model 1 Pancake or pocket carried. Do I consider this to be enough power for self defense? Probably Not. But it is better to be carrying a 2" 38 airweight, than to have a 45 at home on the dresser.
Bob