They have a customer service section where you can email them to ask your question of them directly. I couldn't find any weight/size requirements or any sort of manual that would say. I hope that helps!
I was going to say that I have an almost 14 lb cat, that uses it, but I just thought about it and realized I've never actually seen him in it! I know zakk uses it, but he is not even 10 pounds. We have 5 boxes- one regular tray, a petmate covered, and two tidy cats boxes along with the booda.
I got one recently with the steps up the side for our Bijou who is 7.3 KG (that's 16.09 lbs.). I finally don't have all the litter on the floor in front of the litter container. I believe the Boodas may come in different sizes but don't quote me - worth checking out.
Its so difficult to know what to get! The Booda looks ideal but seeing that if i get one I have to order it from the US (im in the UK and can't seem to locate Booda's here).... I just want to make sure its worth it. Thanks for your input guys!