2x 1920x1200 24" monitors is enough for me. One in front, and one slightly to the right. The available space to the left is needed for equipment. To get more screen estate, I would have to buy a monitor foot allowing multiple monitors to be stacked above each other, but it isn't practical with too large vertical span. I tried rotating the displays into portrait mode, but it is not comfortable. Besides, most IDE are designed for users with monitors in landscape mode.

One thing that is important when increasing the size of flat screens is to make sure that the monitors have good LCD panels with good viewing angle. The really cheap panels have very little vertical viewing angle before starting to invert the colors. On a related note - anyone buying a digital photo frame for christmas should check the viewing angle carefully. Many of then supports portrait and landscape but will look awful in portrait mode unless you view the photo from one side.