Hello! The size hutch you describe is the minimum size for a pair of rabbits. However; it's always preferable to have an attached run rather than a separate one so that the rabbit(s) can choose when to come out. You can buy hutches over runs which take up very little extra space but offer the bun(s) an opportunity to come out whenever they want to. Minimum recommended run size is 6x4ft.
With regards to your chosen breed - are you aware of the problems rex rabbits can have? They are prone to sore hocks because the fur on their hocks is often too thin - especially in summer. They are prone to other problems too - Jacks-Jane on here has several rex rabbits and I'd strongly recommend pm'ing her about them before deciding definitely on some.