Hi~ I would suggest that you just get a normal cage with a tray at the bottom. This will make sure that the poo poo will drop down and its easy to clean it. A normal size cage that you think they got space to move about is good enough. Since they are small, you might wanna let them out for some exercise running here and there~ hehe~ You reminds me when I first have 2 rabbits and end up having up to 30+ of them in my yard.. that was about 10 years ago haha

Try look around Pet Safari for cages, they got lots of selection.. but the price abit high.. If you want cheaper, you can try just any normal petshop will do =)