I assume "LENGTH" means the pin-to-CG length, not the pin-to-PAP length.
It still depends on the type of reaction the bowler is looking for. For the midlane and hard arc reaction, I'd say 1-2" pins, 3" at the most. That will let you put the pin below the fingerline without pushing the CG too close to the thumb. A 2" pin will give you enough movement to swing the CG out and allowing a weight hole if needed. A 1" pin or shorter might be harder to move the CG enough to necessitate a weight hole (not always, depends on the pin location).
SH