Mine will just have a 7ftX5ft shed to share with the geese. As long as its dry, secure, easy to clean out and close to a compost heap (for all the straw and droppings) I don't think you'll go far wrong.
I'd also recommend fencing around it (doesn't have to be a huge area)
There are 2 reasons for this, firstly to keep the fox out (or atleast slow him/her down) and secondly you'll find it easier to get them away at night if you feed them morning and evening in their enclosure.