The General rule of thumb for a coop is:
3-4sq feet per bird.
1 linear foot of roost bar per bird, at least off the floor
1sq foot of ventilation per bird* very, very important, especially in the muggy south east.
I've never seen an eglu that came close. It appears the designers of eglus base them more off plastic outhouses than proper coops. Their only plus is that they can be hosed clean.
For the price, I'd build a tractor coop or find someone to build one.
I think it helps to distinguish between roosting / sleeping space and what is generally thought of as coop space. The feed & water and areas for scratching, running around, are not provided for with the Cubes or Eglus themselves, but instead with the area underneath, the attached runs, or attached (or surrounding) walk-in runs.
The housing itself is for sleeping and nesting only. Considering roost space, there is easily enough for ~6 standard chickens in the Cube, not counting the nesting area, which some use for sleeping too. Sometimes people remove the nestbox divider.
I agree that it would seem the Cube doesn't appear to meet the ventilation common rule of thumb, but it does work well. Surprisingly there is ventilation beyond the visible ventilation windows, so it may not be as deficient as it looks. I've observed that there's ventilation from below, around the poop tray and back tray opening; in each corner of the back access door; and along each upper side seam.
Maybe that extra accounts for the lack of problems with frostbite that most owners have, or it's the particular placement of the ventilation openings, I don't know. It could be the double-wall construction helping too, it moderates the inside temperatures. I haven't had a problem with ventilation for 5 birds (four Buckeyes and for one winter an added Orpington). It concerns me most here in the Northeast for the colder months. I've gone through three winters so far with the Cube. We get -10F and colder every winter (-20F this last one).
In summer mine is located in a shaded but open wooded spot. I clean the poop tray daily throughout the year, so they don't have a build-up of poop and ammonia underneath them in any case. I do have concern for cooling on hot nights, as my Buckeyes are not hot weather birds, and if I were to add more to reach the max that will be my main concern. Some put ice packs in there, or set up a fan outside one of the ventilation windows, pointed in. But folks do that also with conventional coops. I have heard that some who keep their Cubes within very well-secured walk-in runs remove the back door and/or the poop tray, so the birds are roosting in the open air. I prefer the added security of the intact housing.
