There are different scenarios in the life of your cat where you might need a small cat litter box. Generally it is recommended that you should get a big enough litter box for your cat. So your cat feels comfortable moving around in it.
Many cat owners like to start with a small cat litter box though. Kittens learn faster to use a litter box when the stepping in and getting out happens easily. Many of the available models offer a lower side for easy access. Older cats might prefer for the same reason a small cat litter box too.