Up to about 26" tall you can make a lot of tire/rim combinations work. The 275/35/18's I have on my Demon clone are 25.6" tall, but they're 275's so I still had to do some work on the lower front corner of the wheel opening. I know GmachineDartGt used to run 255/40/18's on the front of his car, those are 26" tall and still plenty wide
The further you go past 26" tall the narrower tires will have to be and you'll want them and the further toward the spindles they'll need to be. The further out toward the fender the wheel gets the less you'll get away with for height.
But like autoxcuda said, we need to know more. If you talking about front runners on 15x4's you can go a lot taller than if you're trying to get 255's in there.
