Well more than the screen size, the resolution matters the most...
There are 22" and a 24" LCD with the same 1080p resolution...So there will be no difference as for the graphics performance...
But there are 24" monitors with 1920X1200 resolution. which requires a more powerful card to handle those resolutions...
It is more of a personal choice I would say...some people like to game at larger screens and some people prefer normal screens...