Better idea: I've been jotting "blueprints" at work all morning (yeah, I'm supposed to be working!) and thought that instead of vinyl windows, why not just build a wood frame and then build frames in which to insert sealed double pane insulated glass? The wood frames would insulate better and take slightly less space up than framing in vinyl windows would.

Question:

Are there companies where I can order custom-sized (and triangular) insulated, sealed double-paned glass ONLY on a consumer level?? Any idea whom? I think I can do this myself and save over what a contractor would charge or over ordering a kit.

Wanting to let in as much light as possible, I want max glass and minimum frame. What's about the largest I can go on a single hunk of glass?

Any local glass company can custom make any size and shape window you want. But when it's insulated and tempered the price is high.
The width and lenght of the glass span depends on the thickness. Of course the thicker the heavier and also more costly.
I used insulated sliding glass door panels to cover my portico. Just removed the frames and built wooden frames around them.
Here's a picture but you can't see the roof.
The doors are regular storm doors and the arched and side panels are Lexan.
I'll take a picture of the roof if you want it.
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Found a few pictures of greenhouses made from windows.

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This one is probally Lexan or something but could be easily made from windows.
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This one is just for inspiration.
If you built a solid north wall it would help with the heating sitiuation.
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I like this mother earth solar greenhouse below

Here is a link that might be useful: M.E. solar greenhouse