18
What would we do without the world wide web?
I knew that volume is determined by length (17") x width (11.5") x height (5.5"), but have never trusted myself with this sort of thing, so I went on line. Turns out per that formula, the insert volume is 1,075.25 ci.
Navigating to another site for conversions, I punched in the data and lo and behold, 1,075 ci = 18 quarts.
If I ever find myself roasting a bird that's too large to fit in the GE, I'll just put it in a roasting bag and use the conventional oven. Otherwise, like Keven said, if it'll fit, it's going in the roaster.