its a 6'3" to big for me if im only 5'9"

NO!
You will be glad u have it when its big and alot of water moving in a 5 mil suit. Draw longer lines and more drawn out turns.

 

Make sure your foot placement is correct since you have a few more inches of board. You'll get used to it and if you wanna loosen it up just change the fins.

 

A 6'3 PSB might be a bit big for average surf at your height and weight, but like wonton said, you might appreciate the extra float in your winter suit, in heavy, overhead winter waves. There's certainly something to be said for keeping a "winter board" in your quiver. My standard recommendation for that board would be from about 155 or 160 lbs up to about 190, depending on your age and fitness level, and type of waves you ride.

Switching up fins is a good idea for loosening up a board that feels too big. Three things to consider fin-wise are an overall smaller fin... a more upright fin with less rake... and a fin with more flex. This will help most in smaller, average surf. In bigger waves, you might want to stick to your standard fins, and get used to the feel a bigger board gives in bigger surf.