litter tray size

I have two English spot rabbits and they are both 3.6kg, on average what is the best style and size litter tray for my beautiful babies

I buy underbed storage boxes as litter trays. I have a massive one that 4 bunnies can sit in. Its about
24"x 36" and i have a standard underbed storage box

Get them both from Tesco, easy to clean and if they do stain/break, not too expensive at £4 or just above

Thank you really helped