What size Rio do you have and how much do you weigh? How are skills? Tack, jibe, planing, harness, foot straps, beach start, water start?
Generally, going up one meter (6.5) is a reasonable step up. If you are a big guy, then a 7.0 could be an option. A lot depends on how committed you are to windsurfing as well as how far you want to streatch your $. If you are lucky, you may be able to find a good 6.5 no cam sail that will fit your mast and boom. What is the length of your mast and boom? At some point, bigger sails will require longer masts and booms.
Roger can give you a more detailed answer if you provide a little more information.