Bear with me if you already knew this, but after a few of years with OSX I've only just found it out (accidentally!)
As in finder, where you can use the space bar to quick-look at files, you can do the same in Exposé - hover over one of the windows and hit the space bar to get a full-sized view of it!
As in finder, where you can use the space bar to quick-look at files, you can do the same in Exposé - hover over one of the windows and hit the space bar to get a full-sized view of it!