I have a 24"(1920x1200) acer. Fucking awsome if you're into gaming and have a system to handle it at highrez. Was about my only reason for buying it instead of a normal 4:3
I looked at all the sizes in person before buying and 24" was about the smallest wide screen I would recommend. Because as twisted mentioned the height isn't all the great on the 19-21"
Otherwise if you just want more desktop a couple 19-20's would probably be a better choice. They're coming down in price quite a bit. I might pick up a 19" widescreen to run in portrait mode for stats and shit.