When I use an eastern forehand I'm fine, but when I go with a semi-western my hand starts to cramp up. I assume this means that my grip is too small? It's 4 1/2" right now and I think it should be 4 5/8, but will it really make a difference? I'm going to overwrap it and find out, but does anyone have thoughts on this?
It's actually typical for players to get grip sizes smaller than their actual hand size would suggest. Based on the wrap-around test, I should be a 4 5/8", but I use a 4 3/8". A lot of my former college teammates the same. bear in mind that with overgrip, which I would highly recommend, your grip goes up at least one size.
As for your hand cramping, that probably has more to do with placement or technique. Most likely, you're tightening up on the racquet too much. This can especially happen if your hand is too far Western because you tend to have to pronate the hand a bit more than usual.