I sent this as a PM to MikeE and thought everyone might find it interesting:
I use a 17 inch NEC MultiSync FE700+ monitor. It's 17 inch of course and that's fine for me, I would like probably a 19 inch but no bigger than that. Maximum resolution on a monitor generally changes depending on the manufacture and model. You can always change the resolution of your monitor, it doesn't run at a fixed resolution. I run mine at 1024 x 768 when I am browsing the web usually, but when I am programming I like to set it a bit higher.
A good refresh rate is the highest possible your monitor can safely set. Accept nothing less than 75Hz at 1024x768 and I am sure most monitors will go higher than that. Some monitors have really really good refresh rates at low resolutions (1024x768 and lower) but they drop significantly at higher resolutions (watch out for that).
Dot pitch is size of dots or 'pixels'. The lower this number, the better. A typical monitor has 0.25mm dot pitch but as you start to get higher quality, the number should decrease. Try not to buy something with higher than 0.25mm.
As for buying a good monitor cheap, it really depends on where you live and what certain websites are offering deals. You'd have to check around really.
21 inch is absolutely great for things like movies, games and such but I find it a strain for just using the computer on things like web browsing, programming, chatting, etc. You make the call though.