legless said..
It is not just the size of the fin but the design of the fin as well some fins work really well as twins others do not. The new madd's come with carbon fins.
The other advantage of going for three fins is you can sail it as a single fin as well the board comes with 2 plugs to plug up the side fins.
Been sailing the F2 Vegas twin fins for the last year which are very similar to the original Madd's but I now what to see how 3 fins will work also the longer narrower design should also help reduce the fin size needed.
I have been running Lessacher fins (carbon) in mine for years now. I was able to use twin 23cm fins, but I generally use twin 28cm ones. The 23cms are just too short, and I think any separation from the water surface can get them to cavitate and then thats the end of it.
The 28cms only seem to spin-out when I am trying to drive them hard. Its most pronounced when I am sailing in light winds with a big sail and I try and head too far upwind.
The Hifly fins that I got with my first Madd are pointers and are relatively thin and don't seem to have too much surface area. I broke one, and haven't had the chance to compare them to the weed fins that I run now.