It depends a bit on sail sizes and what type of sailing you'll be doing.
The original 25 Drake Natural is a very good all round fin, covering both freeride type of sailing and wave sailing. If you want to improve freeride type sailing even further, you could look for a stiffer and more upright fin, like a Drake Crossover (a similar fin is in the Gsport lineup); 24cm for 5.0 and up and maybe 26 for increasing upwind and early planing with sails around 6.0.
For waves, particularly in better waves or with sails from 5.3 and below you might want to consider a smaller Drake natural wave fin. I suggest a 22 or 23 as the smallest for this board.