30" strikes me as too large for a single monitor, but two 17" side by side is perfect for me - you can pitch the screens toward each other horizontally a bit so that the screen "wraps" your field of view.
I set mine up about 60 cm from my face and angle them towards each other about 10 degrees. That way the far edges are roughly in the same focal plane as the middle of each screen and the inner edges aren't too far away. With a 30, as Ursus states, you start to have distance problems and if you're too close you'll experience a bit of barrel effect - bad when working with lots of code. That and constantly adjusting your eye focus as your gaze traverses the flat screen creates a bit of fatigue.