I see that there's also a proper Start Windsurfing Forum on the menu, where I suppose the ws teachers can answer you at the best; anyway waiting for them to come here, I have to say that many years ago I've started with a not so far volume board compared to Rio S, and this happened when I was 85 kg. It was and old but tough Bic Calypso, 320x 75 cm, 175 litres. Yes not easy like a debut on a superwide Start, I can imagine, but not so difficult if you have good balance and some patience, especially with some more fall in the water on the first days.
The better would be having before all some lessons in a windsurf center with a Start, if you can reach it around where you live
Compared to Calypso Rio obviously is also really planing board, has more volume in the middle of board, and is much lighter than the plastic Calypso (15,5 kg). And after learning I'd keep it for lightwind subplaning cruising, where daggerboard is a must, or lightwind planing with big sails.
But wait for the suggestions of some of the instructors that use to write on this forum, I'm only a common rider.