For a normal medium pair of rabbits they recommend 36 square feet which one way of calculating it woudl be 6 x 6 square feet or a 6 x 2 hutch with an attatched 6 x 4 run.
If you have an 8 x 4 shed or kennel that will give a total of 32 square feet, therefore you would need to add a large run to this to make it big enough. a lot depends on the size of the giants as there are giants and GIANTS, this would refelect on the size required.