I'll have to give the larger fin a try.
So far, in fully planing conditions, with a 6.6 m2 NXslm race sail, I found the stock fin (41 cm cleaver type) to be too small.
Maybe a 54 cm is a more reasonable size, but with the highly curved deck in the back I found the footstrap positions pretty good.
I think for the width of the board, even with the footstraps inboard, a 54-60 cm fin will work nicely, especially to get the Rio going early.
Also, the Rio's are significantly narrower that the previous Start based extreme width boards. Last years ('07) Rio's : L=101 cm; M=90 cm; S= 85 cm....... 2008 Rio L= 85 cm; Rio M = 80.5 cm; Rio S= 76 cm.
Hope this helps,
Last edited by Roger; 14th May 2008 at 06:15 PM.