If your bunny tends to pee over the edge of the litter box, there are a couple of things to try. A litter box with a raised rim might help keep his bottom inside the box. One with a lid would fix the problem (but some bunnies are opposed to lids). You can also protect your floors or carpets by placing a plastic bag on the floor with a heavy towel that matches your carpet or floor (so it blends in) over it and then put the litter box on top. Instead of cleaning your floor or carpet constantly, you can just throw the towel in the laundry.