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For home use I think 24" is large enough (I have a 24" iMac).

I saw he 27" and the older 24" side by side at a local Best Buy yesterday.
they are the same height, the 27 is just a few inches wider

the extra width comes in handy for having several apps (or web pages) on screen together without having to hide / reveal them in Expose'

the 27" really isn't as big as people think
 
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