book - NEW GREENHOUSE! How very exciting! The bigger the better, I reckon! I think, because you're so keen, you will use whatever space you have and then some! I bought one with a bit of extra height and it's been more useful than I would have guessed to have 'up' space as well as 'outward'!
Also, I bought really thick 4mm glass, which has been brilliant and really 'feels' so much safer - I know you have your DGS around and I'm guessing there may be ball games! Because we don't have little people around, I was really skeptical whether we needed it and whether it was worth the extra cost, but actually I am quite clumsy and I think I would have broken thinner panes already.
Depending on how tricky the design is, I reckon you and your DH will have it up in 2-3 days of reasonable but not back-breaking work. (It took us 2 and a bit days for a slightly smaller one, and we were working out way very methodically through some very comprehensive instructions for fear of making a mistake). So if you ordered it for a time when the weather isn't too bad, e.g. May, you could probably get it up and running without setting anything you're growing back too far?? We found that for ours, there was a considerable lead time on delivery of about 8-10 weeks in the spring, which really threw out all of our plans. I think a lot of people buy them when the weather gets better and there can be queues.
The one thing I really wish we'd bought before we did ours was one of those glass-lifter suction things that makes you look like a cat burglar in Mission Impossible. I reckon it would have been worth it, and not just because I could have fantasized about Tom Cruise tending my tomatoes. 