You need to think about the area in which your litter tray will go, as well as the size of your pet. The depth also needs to be taken into account.
The tray needs to be large enough for your rabbit to comfortably sit at one end, and have a pile of hay next to them (rabbits like to eat as they poop). Something around 40cm x 25cm would be fine for a small rabbit.
If you have multiple rabbits, and you would like them to use the same tray, they will ideally be able to go in different spots. Take this into consideration before buying.
This is a crucial part of the size so deserves its own little thought!
Rabbits and many other small animals back into a corner and lift their tails to urinate. If the depth isn’t enough, this will just go over the edge of the tray. You will be placing a layer of litter and hay in the base of the tray, so take this into account too – it will technically lower the overall height.
But they still have to be able to get into the tray. Some will have no problem jumping into a tray, but it is best to look for a tray which has one lower edge so they can easily enter and exit without injury or causing too much mess.
Most small animals will usually pick a corner to go to the toilet in. Therefore, a corner litter tray is a great idea. It should fit quite snuggly into the corner, and it is a good idea if it fastens onto the cage so when they move, it doesn’t move with them.