If the fin is not curved or just slightly curved, you may find out the aspect ratio by relating the length of the fin to the mean geometric chord. Longer and narrow fins are producing more lift than shorter and wider fins of the same area. Actually, the lift force is growing with the square of the draft. But, simultaneously control problems are coming up under the over-power conditions.
Good Formula fins have an aspect ratio of about 1:8. If the fin is too narrow it takes more time to develop a constant flow pattern and the performance at low speed is decreasing.
If you want a good performance at low speed and on upwind courses you should prefer a longer fin. If you are going for speed, stability, control and manoeuvrability take a shorter fin. But in addition you always have to take into account size of sail, size of board, wind force, wave conditions and body weight.