What sized monitors should I get?

At the moment I'm using a 4:3 19" Acer monitor and a 4:3 15" Dell monitor, with eyefinity (WOOP) on the horizon I was thinking about replacing the 15" one and maybe getting 2 widescreen monitors to go outside of the center 4:3 19" one (I figure more peripheral vision (wider screens on the outside) would benefit me more)

Ok, so, for the heights to roughly line up, what sized widescreen monitors should I get? I could probably work it out but my maths isn't totally great.

Also am I right in thinking monitors are usually 16:10 resolution as opposed to the 16:9 you get on TVs?

Thanks ^_^