The American Rabbit Breeders Association has laid out some [minimun standards of care.][1]

They recommend based on weight the following minimum size:

+ Under 4.4 lbs - 1.5 ft²
+ 4.4 to 8.8 lbs - 3.0 ft² 
+ 8.8 to 11.9 lbs - 4.0 ft² 
+ over 11.9 lbs -  5.0 ft² 

These are the minimums.  I think of these as jail house conditions  Sure you can live in a 4'x6' cell but its miserable. Now if your rabbit will have free run of a larger area most of the time and just having their nest area in the cage then this size is probably fine.  But to your rabbit its cage or hutch is its safe place.  If you can give it a large safe place that is always going to be prefered.

For me the mimimums are at least double the length of the rabbit wide by four times the rabbits length in length and at least tall enough to let the rabbit sit up on its back legs.  That means that except for dwarf rabbits they need 18-24 inch high cages.  

Your rabbit needs a place that is can safely relieve itself.  I find the best option for this is a plastic tray under the cage, and if the rabbit will be running freely then a litterbox in the rabbits preferred corner.  If you clean the litterbox daily and the tray every few days this chore is much easier.  The longer the litter is allowed to sit soiled and the longer the tray goes with out being cleaned you will find the smell becomes more pungent and the more effort is involved in cleaning. If the tray becomes to ripe the fumes from the rabbits urine can actually damage and even kill the rabbit.

Even if your rabbit is in its cage most of the time it still needs time to exercise.  You should get your rabbit out at least daily and allow it to run and socialize.


  [1]: https://www.arba.net/PDFs/CAW.pdf