We've got two of the big size cylinder compost bins at our allotment (not sure of litre capacity sorry but the biggest of those "dalek" ones you can get), and with 6 bunnies (and kitchen leftovers) they both got full after a few months, but that said over time the stuff at the bottom mulches down so there becomes more room at the top again. They need stirring now and again dont they? We got ones with a cover at the bottom so that rodents can't get in to it, as I wouldnt want to hurt one when stirring it. But thats by the by. Basically get the biggest you can get is what I would recommend. Bunnies can poop for England! :lol:
PS We also stick in the hay, straw and megazorb but try to mix it with other 'greens' to get the mix of greens and browns right, so plenty of grass clippings, plant trimmings, any kitchen veg that the buns cant eat, etc.