It's framed with 6x6 rough cedar...
Roughly 16 x 8 feet...
I like the size for our deck/area, but if the area would tolerate it better I would go more like 16 x 10-12ish. That size just wouldn't have fit/looked right with my configuration.
The extra 2-4 feet would allow a little more room to put a bar/counter on the "near" side in the picture... It can (and probably will eventually) be done as is, but it'll be a smaller bar and not as "weatherproof" as it would be with the extra space.
As far as size... With reasonable rain everything stays pretty dry... in Oklahoma we usually get pretty good wind with rain, and taht will blow water everywhere. Overall though, I've been pretty surprised at how dry it stays under there.
No idea how far you could go without a column... these beams are really stout, but I'm sure the roof weighs a lot too!