It isn't the size that matters as much as the resolution. My laptop is 10" 1920x1080... my TV is 65" 1920x1080... but they are the same resolution.
My main desk has the good stuff though. The center screen is @ 2560x1600 res.... 30".
The two verticals on either side are at 1900x1200 each - 24" (tip: if you are going to use a screen vertically; get 1200 pixels instead of 1080 - the extra keeps you from having to scroll left and right reading websites formatted for 1024).
Video card wise: I run the 1900's off of a GTX275 - and then I dedicate a GTX580 to the 2560 screen.
Here's a photo where you can see the screen from that silly mouse review I did:
...and here's what 4960x1900 resolution looks like from a screen cap POV.
Full size image:
Hope this helps.