275-50R15 for the rears is a very universal size as far as drag radials go if you want to go that route. Anything bigger and you might have clearance issues. I say MIGHT because it's not impossible to get tires wider OR taller to fit, but it might mean breaking out the BFH. Keep in mind, though, if you stray too far from a 26" tall tire, your speedometer will be off.
If you DON'T want to run drag radials, it will be hard to find a wide street tire with any sort of a decent performance rating for a 15" rim. A lot of "high performance" street tires don't go much wider than 225/235 in 15" diameter. Now, a tire like a BFG Radial T/A you'll be able to find in really fat sizing (255+), but you'll be sacrificing stickiness and speed ratings. Just something to consider.