Right now I have a 20.1" and thats big enough for me, however I too plan on doing dual monitors the next one im looking at is a 23.1" asus LED monitor. Personally if you like your 19" then no reason to not get another, its all about your preference really I like widescreens for gaming, you haven't gamed until you've gamed on a widescreen IMO
How much are you looking to spend. I bought a 27 inch famous maker (HP) for 240. Gorgeous. Now I can sit back and relax a few feet away. Anyway thats what I did, the only bad thing is that you cant look at stuff while playing your games like you could with 2 monitors.
I have a dual setup 22 inch on my main and a 19inch on my 2nd. I love the 22 inch for gaming and since I work from home doing DSL support the 2nd monitor is the greatest thing ever. I think it all depends on how much your willing to spend and put into it, either way you go I can say this I don't think I could live with a single monitor anymore. Once you go dual monitor you don't go back.
I have a 22 inch and it is marvellous but my brother has a 19 inch and thats really all you need I think. (Heh heh, dirty joke goes here) But as others have said, bigger = better so if you have money to burn or catch an offer, go for bigger.