Model was uploaded from a wide screen (highlight to see shape)
Upload trick of the day. I may be wrong about this but I don't think so... when uploading a model to the 3D Warehouse, your SketchUp window width-to-height ratio determines the ratio of your preview image. Therefore this is one more technique you can use to control how large your model appears in that window.
In the above example, "Kelso Town Hall" by Alistair_M, we can see the model was uploaded from a wide screen.
If you can't see it then highlight the image.
Model as it appears in a standard 4:3 window with shadow settings turned on.
Since the allowed space on the model preview page is always about 4:3 ratio, if you upload an image with a widescreen ratio, the result is that your model will appear very short (like a "letterbox" format with the top and bottom areas unused). Resizing the SketchUp window to about a 4:3 ratio yields a preview that will fill up the space as in the example above.
I usually like to turn on shadows too to give the model a brighter appearance. Check out the "My Favorite Thumbnails" collection for more upload tips.
EDIT: Upon further examination, it seems to me that the preview window is more of a 5:3 ratio.