Recently I've come to the conclusion, that the best grip size (thickness or length, whichever you have the ability to adjust), is that which allows the finger pad of the distal phalanx to comfortably sit on the trigger (center of the distal phalanx), 'pulling' the trigger as opposed to pushing it, without pinching the finger pad of the middle phalanx between the trigger and grip when shooting double action. Also, the pull on the double action shouldn't move or slide about on the finger pad.
Best way to find out the correct grip modification, is to randomly grab your pistol (off the floor, draw from holster, etc) and see which grip configuration give you the most consistent grip which allows for the above finger placement. Then stick with that grip size/modification.
But of course, this is only my opinion...