sizing

the recomened size for a single storey hutch for a pair if netherland dwarfs is

5ftx2ftx2ft

what would a double storey be?

I'd be more inclined to go with the norm of a single 6x2x2 or double 5x2x2. Nethies might be small but they are very active bunnies.

Basically, the biggest hutch you can get is best! The bigger ones tend to have been specially designed by rabbit experts anyway, so they'll be of higher quality and more bunny-resistant than a cheaper mass-produced one, so it'll save you money in the long run.

I would still say 5x2 at the minimum, personally I would still go with 6x2.