I'm not a graphic designer, but I dream of a 30". What I'm working with now is the iMac in my signature, with a 15" 1024x768 LCD stapled to the side. It's funny to think that only a few short months ago, my primary workstation was an 800x600 iBook....
I'm generally working with anywhere between twenty and thirty Safari tabs open on various pages of my Wiki (I use a MediaWiki to organize my writing). I wish I could have seven full-sized (ie, 1 page) windows "in the front" -- I think that'd improve my productivity by 600-700% (no joke -- it's all about parallelism). I don't have the real estate for it, especially since I'm often working with maps and I'm not quite clever enough to implement my own version of Google Maps.... not to mention that I have charts and tables and graphs roughly the size of Rhode Island, a lot of lists, and so on. As it is right now, I usually have three windows -- one map on the li'l LCD, two windows side by side on the 20".
Ideally, I would like to have a 30" screen in addition to the 20". That's my dream 8) Which isn't going to come true any time soon since I'm going back to school 🙄