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I have 21" on my desktop. It fits my needs and the design is not too big for my desktop with papers, books, etc. However, when I bring iMac for showing photo album to my extended family, 21" is so small to watch together with few people.
All in all, 21" might fit for working (assuming you are not too old), while 27" is better if you plan to watch with few people on iMac.
I used to work in 27" few times. A bit too big for me. But the reason I choose 21" is I got very good price and it still fits my needs.
 
It really just depends on your personal needs. My main work monitor is 32" curved with a 27" secondary, and my MacBook Pro 13" as the third. I considered larger for the primary, but am very happy with the 32".
 
Do you have a pic of your setup? This seems like something I'm interested in doing.

IMG_1866.jpg
 
I do. I just bought an 27 inch six weeks ago and was not able to adjust to the size. Now I just bought an 21 inch and I'm using the 27 inch as a TV.
I had a 27 for three years bought the 21.5 next and went back to the 27. Not only is the screen nicer but the 27 has easier to upgrade ram and the specs are better.
 
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