If you *really* want to run usability tests, you can get the actual screen size for each reader and go into a word processor and create a text box of that size. Then fill the box with text formated to your taste. Print it out: that is pretty much how each size screen will display.
For extra points you could do a cardboard mockup of each readers' overall size and paste the printout in it.
(Me, I went as far as the cardboard mockup for my first eink reader to see how usable the button placement we. Screen sizes I don'tt sweat much but ergonomics are important to me.)