What other exercise do they have available? Obviously if they have the run of the shed and the hutch is just for sleeping it doesn't need to be so big as if they are spending most of their time in it.
Also what size are you bunnies?
For medium sized rabbits I would say a 5' hutch plus attached run.
Height is also important, they should be able to stand up on the back legs without rubbing their ears on the top. Depending on how tall your bunny is thats usually 18-24".
A two storey hutch is a good way to get extra space without taking up lots of room, that might be particularly handy inside a shed, where floor space is limited.
Some of the members here have lovely setups with rabbits living free in sheds, with shelves and a sleeping box. Thats a good option and if you already have a shed, cheaper than getting a hutch.
Tam