I think the Nova is a bit wider in the tail than the Rio M.
Tail width (actually rear footstrap offset from the centerline) would have alot to do with
how long a fin span you can use and still have control.
I've used the Rio M with a 60 cm and it was quite good.
I would say that 60-65 is probably more fin than you need unless you are trying to get
planing in ultra marginal windspeeds (i.e. < 12 knots on a Rio M).
Back in the day (early 90's) we used various 70 cm fins in the very early 85-95 cm wide
Formula boards (F-155 etc) and they helped with early planing, but got a bit out of control
in windspeeds > 14 knots.
Hope this helps,