Good to hear from a neighbor, but I'll admit I had to look up Ratcliff. I do like that name.
I think you will probably be OK, but it depends on how you manage them. In my opinion, too many peope get hung up on coop size only. What I think is important is how much total room they have available, whether that is coop, coop and run, or something else.
If you leave them locked in that coop very much, especially if you add the Hamburgs later, you might have problems. That''s probably too small for that. But if they have access to that run most of the hours they are awake, that should be plenty of room.
I always promote giving them as much room as you can stand. It's not to pamper the chickens, it is to pamper me. I find the more room I give them, the less work I have to do. For example, I fully believe the less often I have to clean out the coop, the better. And if I have extra room, I have a lot more flexibility in how I can handle them when things happen. Integration is a lot easier and it is a lot easier for a broody to raise the chicks if she has room.
I don't know of the type of coop your are planning on, a walk-in or one of those elevated ones. If it is a walk-in, I'd make it a little bigger to give myself room to work in there. If it is one of those elevated ones, you need to keep it a reasonable size so you can reach each corner.
Building materials generally come in 4' and 8' dimensions, so it is a good idea to keep those dimensions in mind when designing a coop. But don't forget the roof materials. You can build an 8' x 8' for very little more than a 4' x 8', except you have the roof span to consider. If you are building a walk-in, a 6' x 8' might be a real good size for you. Use 24" centers for your wall studs on the 6' side and use 8' long members to span the roof and give you a little overhang. Depending on what materials you are using to cover the coop and how many shingles are in the packet or how long the metal is, you could maybe get a 6' x 8' for the cost of one sheet of plywood. Of course, that depends on what materials you are using and what style of coop you are building.
20' is a long span for poultry netting. You'll probably need to support it. And 6' does not give you a lot of headroom. I bounce up and down when I walk and that stuff can sag. I like the total square footage, but you might consider making it longer and narrower so it is easier to span and support the netting.
You may notice I'm talking mostly about your comfort and convenience. I think they will be fine spacewise as long as they have access to the coop and run.
