Thanks all for your help with my coop height / design dilemma.
We got the roost in, and the girls are VERY happy to be out of that tiny dog kennel (well, it's German Shephard size, but the walls kept closing in on them... they were NOT happy!)
They are only about 61/2 weeks old, but it has been really warm here even at night so I think they will be fine with a light in case it gets chilly.
We still need to finish putting the hardware cloth on the bottom part, so they are confined to the top for now (4x8') but it is a chicken palace compared to their old digs!
Cute set up! You can always raise the roof another 6 inches or 12 inches by raising the roof line with 2 x 4s nailed to the frame and use hardware cloth around the top extension for more air circulation.