The litter box must be individual so only one cat can use it. Each animal must use its own litter box if you have several cats. The size of the cat litter box must be directly proportional to the cat's because it must be roomy enough for the cat to quickly enter it and have room to turn around, be able to dig in it, and not have to go outside when it wants to do so. A general rule of thumb when choosing a litter box is that the litter box should be as long as the cat is after being measured, including the tail.