I personally use 2 x 21" monitors with a 1680x1050 resolution. I have a pretty good vision and I think it might be hard to use smaller units or higher resolution. Particularly if you optimize your system for best performances, wich means less screen precision.
Especially on Cubase 7 which needs width (as well as height) for the ControlMix, if you dont’ t want to spend your time and energy scrolling up and down and side to side, losing the benefit of the dual monitors.
The same result on a single monitor will request a very large one.
Actually, I’m even thinking about purchasing a third one, to manage the very clever Control Room and the Track List to free space for the tracks themselves on the second monitor (18 tracks with readable parameters without the Control Room and the Tracks selector, instead of 14, for a 1680x1050 resolution). Which means a second graphic card, hoping that it won’t steal ressources to the processor…