One cube tall isn't high enough for a bunny to stand up all the way/stretch out vertically, at least not as an adult.
House bunnies (whether it's one bunny or a pair) should have either 4-5h a day to run around outside their cage or at least 60 square feet of space (a combination of the two would also work).
I specify "house bunnies" because the lifestyle differences of breeding and show rabbits cause their nutritional/dietary needs and space requirements to differ significantly from house bunns. As a result, the standards used by breeders differ from those recommended to pet owners... not because they have "lower" standards - any decent breeder takes exceptionally good care of their rabbits and ensures that their needs are well-met - but simply because what's considered "ideal" for one group isn't always appropriate for the other. For example, I'd never feed an 18% protein pellet to my loafy little house bunnies because that's way more protein than they need (which could cause health problems); breeding/show bunnies, by contrast, NEED that extra protein and wouldn't thrive on a 12-14% protein pellet the way house bunnies do.
At my house, I have a 2x3x5H (in grid counts) condo with three levels (two of which are two grids tall) and I fenced in my whole living room to give them about 120 square feet of running-around space - talk about spoiled!
I'm currently living in a 2 bedroom apartment with my boyfriend until next spring/summer, though (about 3h away from the house) and we have less room here. I made a 2x4x5H condo here; it's in a corner and I used an x-pen (16' long when fully stretched out, which is the length most are) to make an 8'x8' pen around it (providing 64 square feet including the bottom level of the condo) by attaching each end to a wall (I put 2-3 screw eyes in the wall for each end and zip-tied the end panels to the screw eyes). Alternatively, two x-pens would create the same size space - that route costs more but doesn't require a few holes in your walls and would be a better choice for a bunny who likes to chew baseboards.
It's important to remember that pet bunnies don't really fit into the "cage animal" category for one key reason - they don't have exercise wheels to run on. Because of this, they need significantly more space for their size than anything else that lives in a cage.