You're getting giant bunnies so a normal litter tray won't do. I'd get a storage box.
I have the 32 litre underbed storage boxes that are about 55cm long and 35-40cm wide and my bunnies (3.6kg and 2.1kg) fit in nicely, so I think you are going to have to go bigger than that, as rabbits like to eat hay in their litter tray together. Just one of your rabbits will be the same weight as both of mine put together, and one of mine is a big rabbit. You'll find that with most of the things you buy, you'll have to adapt things or buy things not made for rabbits or get custom-made products specially for giant bunnies.
eta - For filling it, I use either just newspaper and hay, or megazorb and hay. You could also use a paper or wood based pellet litter, as long as you use one that is rabbit-safe i.e. non-clumping and nothing added.