I run 19"-24"-19" tri monitor setup. I used to have all three 19", but I found the 24" on ebay for a decent price.
New setup (I moved into my apartment at school and the desk was too big to bother to bring...)
I can no longer stand to use 1 monitor often. I've been running the single 24" when I built my brother's computer and even that feels restricting... So when I get on my laptop and try to multitask...it's annoying. Definitely will never go less than 3 monitors on a desktop ever again. 2 monitors seems weird. There's a line right down the middle, so you can't really center them on you. I tired it for awhile and gave up to do tri monitors. Been running tri monitors for probably almost 2.5 years now. First setup was with 3 17" CRTs though, lol.