- In a decent caliber (I'd like at least 380 ACP or 38 special, although something is better than nothing, I guess)
- Able to land hits at 3-5 yards (a backup pistol will be used up close) fast
- Able to land hits at 7 yards with a bit more time
But one factor stands out that is not usually mentioned: "A pistol is too small if you don't respect it anymore." For some reason, people think its ok to toss a pocket pistol loose into a pocket without a holster or anything. This seems like a terrible idea unless you're keeping a paw on it at all times (for example, walking to your car in a dark parking garage). You have no idea where the muzzle is pointed or what may be getting inside the trigger guard.
People who would never toss a cocked and locked 1911 or a 44 magnum bear gun into a pocket loose (I hope) for some reason consider doing just that with a little holdout gun. I don't understand it.