I thought size didn't matter?
Seriously, like Bill I love my 24" monitor for editing. I didn't have room for a 27" but this one is just perfect.
I've spent a lot of time researching monitors. It can drive you crazy. One thing that you should pay attention to while shopping for a monitor is the panel used. Read this for an explanation of why the panel is important.
LCD Panel Technology - S-IPS, VA, Super PLS and TN Monitors
As you can see, a non-TN panel is preferred for image editing. But if you decide to go with a TN panel monitor don't settle for anything less than 16.7 million colors. In most cases this at least gives you a monitor that can simulate 8-bit rendering.
BTW, this explains why Apple monitors aren't cheap.