how long is a piece of string? depends what you're going to do with it i suppose. i cant tell you what the "best size" is, but i can tell you that its gonna vary.
obviously, off road a bigger tyre will give you more clearance... but it will also change your overall gearing, which might not be good for on the road. numerous other things come into it too.
best method is go out, find a tyre that you like, maybe stick a post on here and say "are these any good" and if they are, go buy them :)