You need to make sure that the door is wide enough to fit a litter box and that there is still ample floor space. It is important not to have a wire floor because these can wear down the fur on a rabbit's feet and lead to skin breakdown and infection. If you end up with a wire bottom cage, you can always put some wood, straw mats or another type of flooring on the bottom. Check to make sure the rabbit does not dig it up and expose the wires again. Your rabbit needs at least 3-4 hours exercise time outside his cage every day.
Some people limit the space to one room.
Others give them an entire floor. Whatever you decide, you need to make sure the entire area is safe for your rabbits. You should either keep them blocked off or wrap them in a protective coating to prevent shock and fire. Keep plants that you don't want eaten out of reach. Rabbits will jump onto couches, tables, and often get into places that you might consider out of reach.