Potamotrygon scobina is probably the smallest freshwater ray which will grow to have a disk diameter of around 12 inches, all other species you can safely presume they will reach a disk diameter of at least 18 inches.
The availability of rays is very dependent on where you live, i have personally never seen P.scobina in a store but P.motoro, P.hysterix, P.leopoldi and P.laticeps are all pretty common. You can expect to pay at least £100/$100 for any species of ray and much more for rarer species.