to be honest, if the hutch is only ever to be used as a base and they have 24/7 access to a strong, fox proof run, then the size of the hutch doesn't really matter. Mine very rarely use their indoor cages which are inside their walk in runs - although I haven't peeped out during the night to see if they sleep there!
edited to say that my walk in run is 10ft x 13ft - split in two with a corrugated plastic roof to keep rain off. There is also plenty of shade when it gets sunny.