What's on the bike right now. Measure your clearances to the stays and forks. Then you'll know how much more you can go. But leave some clearance for foreign objects that sometime get caught in the wheel. And also for the normal flexing of the wheel.

But I'm not certain why you wish to change the wheel just because you are going wider on the tire. More info is always helpful here. If you have some idea about aesthetics or aerodynamics that is part of the reason for your change, then tell us so we can factor that in.

Otherwise I've run some really wide tires on skinny rims and they weren't an issue for me. The ballooned out tires did look funny though.