Take a few PDFs on a USB stick with you if you go see an iLiad. It is the only current ebook reader that supports USB sticks.
You can test the Sony's PDF reflowing on a Windows PC, see
Sony's Windows ebook Reader. This will tell you how well your documents reflow, but not what the device itself feels like. You can get an idea of the screen size by taking a screenshot on the PC, cropping out the 600x800 display pane and printing it on paper at 3.6" by 4.8".