My outdoor rabbits have a playpen attached to their hutch which is 75 spft and I still have a rabbit that always try to find new ways to escape the playpen. While the other one it's happy with the size of the pen. Sort of, seem like the size will never be good enough for them, always want to get out and run in to my neighbour yards or into the forest and play with a cat.
It also depends on how big your rabbits are, bigger rabbits need a huge hutch while smaller won't need the same size. Best it's that both should have space to move around and jump in their hutch. Should be able to lay down and stand up on their behind legs.
For example my single french lop have a hutch of 30 sqft while my lop have only 10 sqft the legal size in my country for a single rabbit (not a size I would had wanted, it's tiny). So just double eveything.
The big hutch I can have more than one rabbits in it and I think that size it's great for more rabbits. At least my french lop love it.
A perfect tip when building a hutch make sure it's easy to clean, one side all doors should be able to be open and make the roof go far out on both the front and back. I missed to do those things when building my hutch, so when it storm the cage will get water in it and I have to go into the hutch to clean it, at least the hutch holds both my weight and my brother in it xD
