Minimum caliber...
Folks, lets keep in mind we are talking
handguns here. All of which are woefully weak in terms of ballistic performance. That said, all if a .22lr is what you've got, it's reliable (this is paramount as many small .22 are very ammo sensitive), accurate, and it is something you are comfortable with... it's your life and no one can make the choice for you.
Frankly, I've seen folks with Berretta (sp?) .22s who were excellent shots and were crazy fast in the follow-up shot dept. (which I think is essential in the lower power range). Not someone I would want to face close-up and personal...
I personally carry all sorts of calibers and guns, most of which are in the medium handgun power range (ie .380, 9mm, .45, .38spl., .357), but when I get my Kahr 9mm working properly, it will probably be the weapon of choice due to its size, but more importantly its concealability (thin compared to some other options)!
I will never disparage another for his selections. We all need to recognize the limitations that come with each firearm and work to minimize their ability to hamper our personal defensive techniques.
For what it's worth, the advances in the .32 are impressive and if a pocket pistol was the limitation, the .32 would be my top choice over the .22 or .25.
God Bless!