space for 20ish bantams and a few regular size hens

Someone might correct me, but I am pretty sure you need to figure at least 1.5 square feet of space for banties and 3-4 for each large fowl hen / roo, of course that would depend on which breed, some are bigger than others. So I would say for 20 bantams and 5 or so large fowl, you didn't really say how many, you would need about 45 square feet of coop space, give or take for larger / smaller but I would rather give you a larger figure than smaller! Of course chicken math will probably take over soon!

The run would be another story, I know it is recommended 10 square feet for large fowl, and I think 5 for bantams so if it were me I would do at least a 120square foot run to be on the safe side.

I hope you get more responses than mine, I have no experience with bantams, but wanted to give an educated guess. Good luck and have fun with your babies!