A large size cat box will work for an English Angora. As long as the rabbit can fit in the box and move around a bit, it should be a good size. The height of the sides doesn't really matter unless the rabbit has trouble getting in due to another issue or tend ot pee off the side.
One thing I would recommend is a grate in to box. Even with a good litter, it can still get in the wool and cause matting or tangles. With a grate, the rabbit doesn't have direct contact with the litter. There are sifting cat boxes that have the regular box and a grate so you can sift out the clumps (for cat litter). You can just have the sifting part in the box and the litter underneath and it should work. You can also make a grate using some wire mesh cut to the size of the box. There are also other things you can use to make a grate.
Also make sure you get a rabbit safe litter. Any clumping or clay based litter is not safe for rabbits. A paper or wood pellet is considered safe and effective. A lot of people use wood pellets, usually horse stall bedding or wood stove pellets, these are cheap and effective. You don't have to buy a rabbit specific litter as long as it is rabbit safe, for example Yesterday's News has a rabbit litter that is the same as the cat stuff but more expensive.
