The smallest would be some of the Kahrs, Seecamps, Kel-Teks, Detonics, and other very small handguns. (Despite what some may say, the 9mm is a perfectly adequate defensive arm and has much going for it in the way of inexpensive ammo to practice with and low recoil. Good bullets and proper technique and tactics will do more for you than a bigger, more powerful caliber.) Theses are small but for many they are too small as they do not fill the hand very well. The real question is what is the smallest/big gun that you can carry comfortably. That only you can determine for yourself and much is dependant on holster and type of carry plus your body configuration. If you can, go to a range that has guns for rent and try a few out. I would stay with single stacks as they are usually thinner which aids in carry and concealability. Then pick a couple holsters and try them out, often the range has a shop that will let you try some for yourself. You will go through several holsters before you find one that you will ultimately prefer. Don't go cheap in holsters, you get what you pay for. A good holster will generally start around $40 and go up to $100. I carry an officer's sized Springfield 1911 with double stacked magazine regularly inside the waistband without anyone being the wiser. I can carry a full sized 1911 or the 92F this way and the Beretta will occasionally print as the grip is fairly wide. With a belt carry with a long untucked shirt or shoulder holster, I can comfortably carry my Ruger Super Redhawk with few realizing what it is and is fairly comfortable.
