IMHO, my iLiad's 8.1" screen is more enjoyable to read, than the 6" screen on my Kindle v.2. I like the larger screen better for text, and it is, of course, better for images.
I am currently reading The Bible Unearthed by Finklestein (
http://www.amazon.com/The-Bible-Unearthed/dp/B000FBJG86) and the timeline tables and site drawings, which are for practical purposes unreadable on the K2, are noticeably better on the larger iLiad. (Very well written book, BTW :-)
Up to 10", IMO bigger is better. I am waiting to see what the new 9.7"-ers are like, when they start popping up later this year/early next year, and unless prohibitively expensive, I'll probably get one.