bepsf Depends on the house:

Some would have structural supports within the window wall between the window units.
Some have roofs that are cantilevered with steel embedded within the structure.
Others have structural columns inside the room away from the curtain-wall of windows (Most modern High-Rises are this way)

This one appears to not only have a structural column in the photo here - but one at the corner of the roof outside the envelope of the house itself as well:

http://www.suttonsuzukiarchitects.com/res.html?14

CoorItalia The most common solution is cantilevering. In this project we worked on the windows are in cold rolled bronze which added additional structural support

DecoDesignCenter.com Normally one lite french doors come in either 80" or 96" heights and different widths such as 30", 32" or 36". Cost depends on glass type and finished or unfinished.