I used to carry a NAA .22LR mini revolver, after firing it I had doubts about it's use as a defensive pistol so I sold it. I purchased a Kahr CM-9 that is now the smallest gun I carry. To be fair though I carried the .22 often and it is served its intended purpose.
I find that I can hide my S&W 638 revolver in cooler weather, but in warm weather when wearing shorts and a t-shirt it bulges and prints no matter where I try to hide it.
So in hot weather, I carry a small .22 LR that hides well in my front or rear pocket. I have a S&W 61-2 Escort that is thin, flat, and very light. It hides real well.
I also like to carry my Beretta 21A for the same reasons.
One of these days, when I can afford to buy one, I'll get a Kahr P380 and then I can carry it year round.
The smallest I carry is a 642 s&w airweight. While not the most pleasant gun to shoot its small enough that I can carry on my ankle, iwb or just throw it in my coat pocket.