My recommendation would be a Glock compact size, any caliber of your liking is fine...9mm, .357 sig, .40 are all great calibers. This size of gun packs some nice firepower, is utterly reliable beyond anything you can imagine. It is a bit larger than some of your super small guns. If you truly wanted something smaller, I would have to recommend a small snub nose revolver. I think reliability is the utmost importance in a self defense gun and some of the smaller semi autos are very particular about ammo and such.