Dual screen gaming setups are usually used so that you can have your game running full-screen in one monitor, and keep an eye on your social media and instant messaging programs on the other. If this is the kind of thing you want to do, then go for it.
The problem with gaming on the PC at the minute is that people always seem to assume that a bigger screen means better gameplay. Let me assure you that this isn't so. Your typical 19" monitor is likely to be full-HD - with a resolution of 1920x1080 and a PPI floating around the 100 mark. However, your average 24-27" gaming screen is likely as not to be of an identical resolution, with a visible degradation of quality, because as everyone knows HD is all the rage, and why would you ever need a higher resolution than that?
So if you want to go for one big monitor, be sure before you choose that you're not going to buy something that's actually of far worse quality than a smaller one... after all personally I would be disappointed with a 27" monitor that didn't have a resolution of 2560x1600 - keeping the ~100PPI sharp image instead of a terribly scaled 1920x1080 image.
Personally, if I was setting up a new desktop with unlimited finds, I wouldn't buy a monitor larger than 24" anyway. The extra size just isn't worth giving yourself a sore neck trying to see all of the screen at once.