Size is rather meaningless, it's resolution that determines performance.
1. Pixel Pitch - The human eye can see individual pixels starting at about 96 ppi (pixels per inch) ..... go much below that an image will look grainy.
http://en.wikipedia.org/wiki/Dot_pitch
For 1920 x 1080, ideal pixel pitch results in about a 23.0 - 23.6" monitor
2. Colors - This is different things to different people..... for photo editing for example, you want a quality IPS panel such as a Dell Ultrasharp for accurate face colors for example. However, games are not intended to be real life....they are designed to be larger than life. Designers also take into consideration the "most common screens" out there much like those who mic music tweak recordings to the most common devices out there..... typically boom boxes and smart phones..... as such, when you play a lot of popular music on hoi end audiophile equipment, it sound horrible, the low and high ends being greatly exaggerated because the "most common devices" listened to can't reproduce these sounds well. So when you look at a game on a fancy Dell $1,000 IPS monitor, they tend to look a bit flat and "fuzzy" on the edges between color changes. We have 3 gaming boxes here .... Dell Ultrasharp as well as an Asus 120 Hz and 144 Hz and if there's 2 people playing, rest assured that the Ultrasharp is the one sitting idle. It is the one everyone wants to do their photo projects on.
3. Lag, Lightboost, 3D - 120 / 144 Hz does a great job of eliminating lag and ghosting and ot's needed for 3D and Lightboost or G-Sync. For Lightboost info, go here:
http://www.blurbusters.com/zero-motion-blur/lightboost/
To see Ghosting ..... tftcentral has great material on this
Asus VG278HE 27" 2ms G2G Chi Mei Innolux TN Film +144Hz (Trace Free = 60)
Dell U2713HM 27" 8ms G2G LG.Display AH-IPS