I've found that a handgun grip really fits my hand if, when I am holding it to fire it, with my finger on the trigger, I can easily insert a pencil down inside the second knuckle (between the third joint on my index finger and the slide).
That way I'm using the pad on the first joint for the trigger pull, and not having to stretch to reach it.
If the pencil is too tight, it means the handgrip is too large for my hand.
Make sense?