After a little bit of testing, the preview appears to use a screen width of around 1000 pixels, so it is very likely it uses 1024 as a base.
I checked Wikipedia, which uses a fluid layout. Judging by the scale of the thumbnail it used around 1000 pixel width (you can check more accurately yourself if you like).
I checked CSS-Tricks.com which uses a responsive layout, and the thumbnail shows the second version down (main content and one column on right). That layout appears in the range 920px to 1200px.
For fixed-width designs, the screenshot is taken at or above the fixed width and cropped to that width. I don't know of a design with a fixed width higher than 1024 but if you do, give it a try!