Size, caliber, weight varies on the clothing, temperature, and where I am going, etc. Those things can mean that I might carry anything from a 4 inch model 29 to a NAA 22 mag mini revolver, or maybe a lightweight commander to a 32 Kel-tec.
What matters on whether a gun makes the grade or not is reliability, reliability, reliability in that order. Shoot-ability, Weight, safety features, and lack of stupid modifications to original designs, all figure in to favorites, but it's unreliability that gets a gun discarded quicker then anything. I have no use for a gun that has any reliability quirks, period.
However, I don't have to shoot one several hundred rounds before I will carry it. If it shoots a couple boxes of shells without failure, I feel pretty good about it, but if it hangs up even once, then it's going to get a much more extensive test. But there are other things to watch out for. Like can the magazine release be inadvertently activated too easily. Or does the safety operate in a fool proof way or not if it has one.