Go 27
I upgraded from a 20" G5 around 7 months ago. At first I was going to go with the 21.5" since it was already bigger than what I had. Last minute I saw a 27" refurb on Apple store and for the value it didn't make sense not to buy it instead, and after using it I am glad I did. Like someone else said, you'll never hear someone complain their screen is too big. I've set this computer up at eye-level just like my old one and have had no back pain like I used to with my previous computer since I don't have to look down to see my screen, it almost fills my entire field of vision - it is great! I also checked (since I was putting it up on a stand) and my new 27" only weighs a couple pounds more than my old isight G5.
I did think that a 27" screen was obnoxiously huge the first time I took it out of the box, but if Apple made a larger screen iMac it wouldn't upset me.