Originally Posted by mim
I do not feel stable enough in light wind conditions (8-12 Kt). I am 90kg of intermediate skills
if 8.5 would remain your largest sail you might have to reconsider your wind range. The iS133/W85 matches a 10.4 very well but a 9.5 works well too (just a bit more work and a bit slower) (in above wind range).
If you really want fun (planing all the time and "speeding") in the 8-12kts range you would have to go for a FU155 Technora (intermediate skills == you WILL have non voluntary contact with your board == if your choice is the iSonic keep in mind its made of wood -> it WILL break).
I haven't tried the FU155 just extrapolating from my iS133/W85 and FU133 (width and especially tail width)