rosesared I have had my north facing one for 15 years and still love it. Sun in the morning for breakfast, and in the evening, good light all day but rarely too hot even in the summer. My advice would be (apologies for repeating others' points):
Avoid greyed roof (north facing wants the light, not to cut out the heat), but as nfk says, definitely have roof ventilation
Choose glass for the roof if you can, it lasts much longer than polycarbonate and doesn't sound like a drum whoever it rains
Tiled floor is great for in and out of the garden, but very chilly in the winter - if you can afford it, I would now choose underfloor heating, which wasn't really an option when we got ours.
Ours is 8 sided, with number 8 being the house. I think now I would choose rectangular for easier use of space, because we use it SO much. We have folding, sliding doors into the house so it can be completely opened up for parties, and even just when the weather is good.
As others have said, definitely as big as you can.
I would just say yes, yes, go for it!