Players often ask how thick their badminton racket handle should be: how many layers of grip tape should they add?
I don’t know of any formula for calculating your ideal grip size. To get the right size, you have to experiment.
Your grip needs to be big enough so that you can hold the racket comfortably with a relaxed grip. If you find yourself needing to clench the racket to maintain control, then your grip is probably too small.
Your grip needs to be small enough so that you can make good use of grip tightening for power. If the grip is so big that it completely fills up your palm, then your grip tightening will not be effective. Having too thick a grip will also make your grip less flexible.