You would need to get a Lessacher Design Duo 28 (or if you don't have weeds, try the 30 deg/ LE Rake Speed Fin).
These are not your normal fins.
They feature Wolfgang's "assymetrical foils".
There is a convex (positive) foil at the top on one side and a concave (negative) foil
on the other side, and then about half way down the foil the covex/conve foils are reversed to the other side. On a weed fin, this means that any weed that lands on the LE of the fin and is symmetrical (balanced on both sides and the kind that really sticks and causes spinout problems) the asymmetrical foil causes them to "flap" on the concave foil and come off easily.
Something about this "asymmetrical foil" system prevents alot of spinout found with purely symmetrical foils.
The guys are really going fast on them, and I will back up Wolfgang's claims on the weed fins as that's been my experience. Try a 9 m2 + formula sail on a 40 CM Lessacher Formula Duo Weed and it will make a believer of you.
The 28 cm is only too small if you "jump on it" too soon, before the water gets going by fast enough.
Otherwise, 28 cm on a slalom board can be very fast, and spinout free.
Only way to really see how good they are is to buy one.
For weeds.... get the 45 deg LE Duo 28.
For Speed.... get the 30 deg. LE speed fin 28
Suffice to say they pretty much do not spin out and they are very fast (check the number of GPS Speedsurfing sailors using them in Holland and elsewhere.
Hope this helps,