18,1 and 19"
A quick search on any store will make clear most monitors sold today are 22" LCD screens.
24" is an exception, and 30" I've never seen.
Plasma is not interesting due to it's big grain.
I'm better off with a 24" monitor a little closer to my eyes, than a 50" plasma with the same resolution (at least for my personal computing experience).
When watching HD movies with a bunch of friends things might be different.