Just got mine, dimensions on the tray are 26" x 12". It did fit a mouse pad and full size keyboard, but that was barely so I switched to a space saver keyboard (which I actually prefer and usually use over on my better computer, heh).
My tray was a little crooked at first, but then I realized I had put one of the screws in askew, took it out, put it in straight, and all was good. Pic (I need a better camera next, I guess ;p):
http://users.rcn.com/dnanek1/new_desk.jpg
I noticed Staples had a model that was almost exactly the same. but made of wood for only 70$ (and not on sale), probably why the glass one doesn't sell so well when at full price. ^^ Neither has drawers or anything which probably contributes to the low prices.