Not sure why you're so salty about
@Icaras' comments. They seem perfectly reasonable and in the context he's presenting them, I think he's correct. He's saying, all things being equal, they're the same resolution and will present text at the exact same sizes. Sure, you can blow up the text size on a 27" monitor, but you can also do that on a 21" monitor too.
If you don't need the extra real estate, a 21" iMac monitor should be just as good as a 27" iMac monitor regardless of your good your eyesight is.