There's more info that's needed here to answer that question, and there are a ton of threads around this subject. Actual tire height, width, offset of the wheels, what amount of work you wont to do to stop any amount of rubbing (sometimes only a little "tie back" is needed) all factor in to what the "biggest" is you can run.
Generally, stock offset wheels, 2" level, you can usually go up to 34". RC says 295/60R20s and RL says 285/65R20.