I struggle with this issue. I am a firm believer in carrying a service caliber or above for primary self defense. I've heard/read all the arguments about this caliber and that caliber and shot placement, etc. I don't trust a smaller caliber to stop the worst type of attacker (crackheads and meth heads hell bent on killing me or my family). Personally, I won't carry less than 357 mag, 10mm or 45acp. That's just me.
As for my wife, I wish she would carry SOMEthing. The purpose of carrying a firearm is the stop the threat, not necessarily kill the threat. I want her to have the best chance possible to stop the threat. I personally don't feel a .22 is ideal for this. It may eventually kill the attacker, but how long with that take? Will he know he was shot and stop the attack or will he carry on and do what he set out to do? I realize this could happen with any size bullet, but bigger and deeper is better IMO. I would HOPE she would opt for at least a 38 snubbie. Something that has less chance to malfunction, no safeties to be concerned with and can be fired while inside her purse. She has shot a number of handguns in various calibers, and believe it or not, she preferred shooting a 457 (45acp) i used to have. Heavier gun with less felt recoil. She shoots my 360 loaded with 38sp really well and has no complaints about recoil. This would be my first choice for her, and hopefully her first choice as well.