"Generally", the safest tire width shouldnt exceed 3" (inside AND outside of the tire combined) of the wheel. An 8" wheel should be fine with a tire of a width around 11.5". Many shops that do tire mounts won't even attempt it and will warn you against it. Many will do it without a word. Having an 8.5" wheel and a 285/60 tire (in my case) works out quite well.

It is good advice to know your size before you go buying though. Nothing like buying too large or too small and then finding out later that you could be in a bit of a hitch for it.