Do you have any idea what year model the Rio boards you are looking at are?
If you can get the serial number (it's under the nose on most Rios) we can figure out
which version of the Rio you are looking at.
The early Rios were simply Start boards with different graphics, so they would be shorter
very wide (90-100 cm). At your weight the Nova 160 might be a better choice than an early Rio L or M..
The later Rio's are much better as they have alot more glide and work as a large shortboard significantly
better than the Start based Rios.
The Rio M should be good for you if it's the one that is 85 cm wide.
Check the archives on this webisite if you want to see the differences between the 08-2010 Rios and the
earlier Start based Rios.
Hope this helps,