What is the best monitor size for the 250gts in sli?

Resolution is what's important not size.

^+1. SLIed GTS 250s should handle 1080p rather well. Those monitors are usually 22-25" and probably what you should be looking at unless by 30" you mean and HDTV rather than monitor. Those would work fine as well for gaming/movies but I wouldn't want to use one as a main monitor.

For the 24" size would 2 or 3 displays be to much in order for good gaming res?

Are you talking about gaming on 2-3 24" displays at once?