If you want the standard CG-in-palm, pin-at-7:30, then go with the pin length that puts the pin as far as you want it.  Like all other balls, 3" pin with 3oz top weight will hardly ever be "wrong".

There are other full roller layouts, but the dominant one is what I gave.  If your driller feels like that's the only option, I'd just get a 3" pin.  3 1/2" will give you something like a leverage layout.  2-3oz of top weight won't require you to drill the fingers or thumb extra deep and you won't need a weight hole.

SH