It depends on several things.
Most important are: you're weight, rig/sail sizes, board (type size volume & width), fin type (race or freeride) and water/weather conditions.
Fore example, I use:
- Starboard futura 133L board 76,5 cm Tail weight 49,1
- weight 92kg
- flat water / little choppy conditions
- Drake race fins: 46/ 48 and 52 cm
- Rig sizes 6.5m^2 / 7.5m^2 / and 9.0m^2
- Don't go 1 or 2cm maximum bigger than tailsize (+/- 49cm in this case).
- 2 up to max 4cm between several fins is enough.
Actually the 52 is a little to big but don't forget that I am 92kg.....
This fin brings very early planing in light wind conditions.
For some info see the links below:
Race-fins gybe less easy (less maneuverability) but plane early and have a higher end speed. Freeride fins are bended/rounded more and gybe smoother.
Its a personal........it depends on what you prefer.
Try some..... before you buy one.....