You can get a 225/50-15 tire on there but it will be bulging out a fair bit. I wouldn't do it unless you're going with something with a very stiff sidewall.

Most tire manufacturers list a wheel width range for each size of a particular tire. There's the "measuring rim width" which is the wheel size that you have to mount the tire on to get the quoted width spec, and then a range of widths that the tire can be reasonably mounted on.