RE: Rio Vs Start
The Rio and the Start are really the same board (hull shape, volume, etc.) with different graphics and the Starts all have the roller wheel on the back.
So, if you want to "roll" your board to the water, get the Start.
If you normally carry your boards to the water or if you have a wide stretch of soft sand (where the roller wheel kinda "bogs down") then get the Rio.
At your weight, I'd suggest the Rio L (Large) with 225 liters of volume.
I had both the Start Large and a couple of Rio M's in my demo/teaching fleet this year and while I did "roll" the big Start to the water a few times,
the roller wheel was not a real important feature for me. It works, but if you want a roller set up, the optional "Clipper Wheels" are nice and will work on any board with a Tuttle fin box, plus the Clipper Wheel setup is
removable so it's not in your way when sailing.
As far as what sail size to use, you could look at some of the sail loft websites and see what their recommendations are, but my guess is that you need a bit more experience at your particular sailing site to "figure out" what the difference between the wind on shore and the wind out in the middle of the river where you sail is going to be.
Also at your weight, on a big wide board like the Rio L or Start L you are going to be using sails about 1.0-2.0 m2 larger than most other recreational sailors.
Get an adjustable outhaul, on a larger rig (say 8.5 min.) and you can "adjust" your sail size once you get out to the sailing area.
An 8.5m2 rig (with an adjustable outhaul) should work for a fellow your size in 14-20 knots. If you are sailing in less wind, then a 9.5 m2 or even a 10.5 m2 free race sail would seem to be just the ticket to get you planing early and give you max. range.
Hope this helps,