Summarize:

Make sure there’s enough room inside the cage to allow your rabbit to move around with ease. As a general rule, it should be able to stand upright on its hind legs without its ears touching the ceiling. A cage with at least 32 square feet of interior space (or about 3 square meters) will usually be big enough to accommodate most average-sized rabbits.  Larger species like English lops and Flemish Giants may require a cage as large as 44 square feet (4 square meters). You'll need to go up to the next biggest size if you plan on keeping more than two rabbits. Rabbits naturally gravitate toward dark, enclosed spaces. For a few extra dollars, you can buy a partitioned cage with separate rooms or levels that will allow your pet to enjoy a little privacy. That way, they'll be able to get some peace and quiet whenever they please.  A partitioned cage will run you quite a bit more than a standard one-room model, but can be well worth it for the added internal space it offers. Regardless of which type of cage you choose, make sure each rabbit has their own shelter or space to hide. A private place to burrow provides a healthy outlet for your pet to cope with stress. It’s possible for rabbits to get snagged or develop painful sores when standing in cages with exposed wire bottoms. Cages with solid bottoms have the added advantage of being easier to line and clean. If you already have a wire-bottomed cage you want to use, slide a flat piece of cardboard or scrap wood over the bottom to give your bunny a more comfortable surface to stand on. The door of your rabbit cage should open wide enough for all the necessary accessories to fit through without difficulty. This includes your rabbit's food dish, water bottle, bed, litter box, and any toys they'll be playing with. And, of course, it shouldn't be too tight a squeeze for your rabbit itself! Some cages have multiple entrance points, like an extra side door or an oversized hatch in the top, which can make inserting and removing various items easier. Rabbits are energetic creatures, and don't like to sit still for too long. For this reason, most of your bunny's cage space should be devoted to play and exploration. Ideally, it should be able to take 3-4 full hops from one end of the cage to the other. Being able to move around freely will keep it happy and healthy.  In a standard 32 square foot cage, only about 8 square feet is for feeding and sleeping.  Add a few simple exercise accessories like balls and boxes to create a miniature obstacle course for your rabbit to run.
Purchase a cage spacious enough to comfortably house your rabbit. Invest in a subdivided or multi-level cage. Select a cage with a sturdy plastic bottom. Choose a cage with a large door. Make sure there's ample room for exercise.