Depends on how large their run will be or if you will free range them. Also if you get snow or lots of rain, most chickens prefer to stay inside the coop during inclement weather. You don't want them picking at each other if they are all stuck inside the coop together.
I would like to have at least 3 sq ft of coop space for adult LF birds. I don't have bantams but I would think they should have at least 2 sq ft coop space. I *think* I remember seeing that there should be 10 sq ft run space per LF birds, I would assume going off that measurement that bantams should have 8 sq ft each. I free range, so I don't know the run requirements for sure.
So mathematically, here are your numbers
2 LF x 3sq ft = 6 sq ft
8 B x 2sq ft = 16 sq ft +
22 sq ft minimum coop
so if you were to get a 4x6ft coop
4ft x 6ft = 24 sq ft
so a 4x6ft coop should theoretically suit your needs with a min 84sq ft run (6ftx14ft) if you don't allow free range.
