Its a difficult answer to quantify but ;;;
In any given wind speed a heavier sailor will require a bigger sail and consequently bigger fin ? (Easy ???)
But for any given sail size a heavier sailor is going to be a) probably going faster b) using sail in stronger winds (definitely faster) c) probably using sail in rougher conditions ; all points leading to a smaller in.
I use a 7 metre in upto approx 25 mph.In these conditions (on flat water) Because of board speed I can get away (benefit?) with a 32cm fin.
Daughter (much lighter) uses same sail in 15 mph and copes (benefits?) with a 42 cm fin. (On same board too)
General rule to give starting size is x5 +3. (Sail size in metres ; muliply by 5 and then add 3)
(ie for a 7 metre would be 38cm)
But its only a guide.
If you need better upwind; go bigger.
If you need more control; go smaller.
If you are going for Vmax go as small as possible. Probably)
Generally use smallest you can.
Its worth experimenting. A fin change is wy quicker than a sail change and n do job.