Yeah for 6,7 it seems to be too small - however we will be really flexible with the fin sizes... as they will be custom made carbon fins - at least for the beginning.
However, it depends on how you windsurf... if you do spocks and grubbies and are already trying switch moves and stuff, the 22cm fin should be ok.
Maybe 23cm but i wouldn't take a bigger one for that purpose.
If you are more freeriding on the 6,7 and do jibes and old school moves like 360s and so on, then you could or should take a bigger fin, because it lets you plane earier and carve more stable.
For 5,8 and 110l board i would say 21 or 22 cm would be good.
I didn't try the Whitewater MX but i think it is quite wide in the base...
I think that makes it a bit slower... our fins will be superfast and as far as i tested till now have super easy planing characteristics.
If you can wait till January 2009 I can give you more info on our fins...
If you need a fin right now - look into the MFC Freestyle for more radical freestyle or check for a freemove fin with 25cm maybe if you do more oldschool freestyle and jibes and tacks.
All best
Rossi