I'm under the impression that 21.5" has normal size fonts. The larger 27" screen has much more DPI, which compresses more "dots" into the same inch of screen. Which means, the font size (like menus and icons) is much smaller. Thus, weaker eyes (like my older eyes) must get closer to the screen - to read the fine print text. This means the larger size 27" is even more harder on one's eyes. Younger / stronger eyes can take much more stress then weaker eyes.
For me, I ordered the 21.5" screen. If current version of OS would allow adjustable font sizing (like Win XP / Win 7), I probably would have purchased the 27" screen instead. For now, we can only use what is available.
As a suggestion, visit your local Best Buy / Future Shop store and "ask for a chair". Seriously.... Try each of their on-display iMac machines while sitting in a typical chair. After some time, then decide which screen size is best for you (and your eye comfort).
Hope this helps...
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