I size to a 4 1/2 and that's what I play with.
I've played with a 4 3/8 grip size and the racquet would spin around in my hand on certain off-center shots. I also played with a 4 5/8 grip size for several months and developed severe tendonitis in my playing hand (required 3 months rest, steroids and physical therapy).
USTA computer-rated 4.5
4 matched Wilson Steam 99's / 4 matched Tecnifibre TFight 325 TPs