i dont think you can really increase the size of the hands since the main muscles can control your fingers are based on the muscular area of your wrist by your elbow.
Typically, if you training comprehensively, ie back and legs and arms etc.... and not just chest, then its normal for the wrist muscles to be proportionaly big.
If however they are still lagging behind, there are 3 exercises i know of that may be of hold.
1st: lay arms down on your legs while sitting, and effectively bicep curl with a bar... but with your wrists
2nd: as above but wrists laying down the opposite way
3rd: hammer curls
hope that helped