Well, the minimum size really depends on theadult size of the rabbit. I have a dog crate for my dutchthat is 42.5" L, 28" W, 31" H and it has a shelf in it. It'sa good size for him. Dog crates work nicely for littertrainedrabbits. I'd say that if you want something prefabricated andwith a solid floor you should check out dog crates. I gotmine (an XL) for $60 brand new at Big Lots. I've seen themfor $10 at garage sales but of course I couldn't find any when I wantedone.Some modifications like urine guards and such might be needed forrabbits that aren't littertrained yet. It's also easy to puta shelf in them for extra space, as they have the vertical room.
If you are looking at "rabbit" cages, make sure that they are tallenough for your rabbit to sit up comfortably. If my dutch satin my nethies' cage, which is a pet store rabbit cage, his ears wouldalmost brush the top. If he stood up, he'd hit his headpretty quickly. As far as the rest of the dimensions go, IMOthe cage should be big enough for the rabbit to lie down stretched outon the wide way and room to take at least several hops the longway. Don't forget to leave room for food/water bowls, litterbox, and toys. There are rabbit cages that could fitsomething bigger than a nethie, but all the ones I've seen are over$100. That being said, I have also seen "rabbit" cages that Iwould not put a golden hamster in. I get a little fussy oncage size.
I do not have a NIC cube cage, but I have seen someone use the plastic mats for office chairs on the floor.