Q. I am not all that sure of the proper size grip. I often experience cramps in my hand and I am wondering if my grip size is contributing to it. I know that in the old days the large finger was used as a guide. Is that still the best guide for determining grip size?
A. Most of the research on gripping involves hand tools and not tennis rackets, so most of the wisdom is still rules of thumb (no pun intended). In an eastern forehand you should be able to get you thumb to cover up some of the nail of your middle finger. The smaller the grip the harder the hand and forearm muscles will have to work. If you are getting cramps you might explore increasing your grip size slightly.
This can be experimented with easily with tape and overwraps. You might also do some grip exercises and not grip the racket so tightly during play. The grip should only firm up at impact. The belief that a very firm grip will increase ball speed is a persistent myth. Excessive grip pressure might actually slow down your stroke and may be contributing to your problem.
Q. Is there any evidence to suggest that one grip style puts a player at a greater risk for injury compared to others?
A. It appears that the extreme western forehand, because of the positioning of the wrist/hand, places a greater load on the wrist and you do tend to run a slightly higher risk for injury. Every single junior player I have seen with wrist injuries has used, on the forehand side, a big western grip. I think, once again, it's almost an adaptation - that wrap-around motion (of the western grip) - because that is the only way they can get the arm into a position where they can pronate. That position is not necessarily going to hurt the wrist, but I think it's an indicator to the body's attempt to compensate for something else that is wrong.
As for the backhand, the problem with the 2-handed backhand now is that everyone is going to the backhand style where the left arm becomes the power arm. It's kind of a guided one-handed backhand. Well by definition, that left arm is weaker - it's supposed to be non-dominant - and to try to get that extra power you change the wrist position and place added loads on the wrist. Once again it's not necessarily how you hold the racket, it's what the arm and racket are doing when the arm is in that position; it is the combination of the wrist position and the load the wrist experiences.
I wouldn't worry so much about the grip, except for the extreme western forehand, but I would worry more about how the racket is positioned as the player comes forward into the shot.