What those of us who have responded really need to know, to make a good recommendation that almost certain to work for you is the width of the board you are looking at to use with this fin, and the range of sail sizes you think you may be using.
Since you are 57 Kg in weight, you can sail a 100 liter board that's < 60 cm wide in even
12 knots with a 6.5-7.0 m2 rig.
For this setup (57 Kg. 6.5 rig <= 60 cm wide board) I would think a 32 cm Lessacher Duo weed fin would be ab out the best you could get.
If the board size decreases and the windspeed increases to above 20 knots, then a 28 cm Lessacher Duo would be my call.
Do not buy a cheap weed fin.... and remember, weed fins have far more area than the fins that are talked about in the speciifcations so you can run probably 20-30% smaller.
The cheap weed fin will most lilkely be slow, give you lots of drag, and very little upwind performance. The Lassacher duos won't have these issues.
Hope this helps,