There is another thread ("N00b questions") where issues about readability and size were/are being discussed. There is some good advice there from various people. Here's an edited version of what I said:
i) Portrait view. This is about 56% of the paper copy. I can read this, but I wouldn't want to read for more than an hour or so at this scale. If you print this page to A5 (e.g. two pages per view) you get about the same size.
ii) Landscape view. This is about 80% of the size of the original paper copy version (but of course you only see half the page at a time). If you use the version of ipdf with the fullscreen patch then that gets you up to about 90% of the original size.
So, you could try printing out a page or two, first with two pages of your pdf file to one page of A4. That will show you what it would look like in portrait view. Make sure your printer is good though. The iliad renders output at that size better than our office laser printer does on paper.
Then try printing at 80% or 90% of the original size and fold the page in half. That will show you roughly what you will get if you view the doc in landscape mode.
Before I bought my iliad I got a piece of cardboard and cut out a window the same size as the screen and then read printed journal articles through it to see if it worked for me. It did, I bought one, and I'm very happy.