It's the resolution and not the screen size that determines the load on the GPU (right?). So, you could go 17" if it wasn't for the portability issue.
I'm a big fan of my Macbook Air - I got the 11.6" screen, but the portability is outstanding, it's lightweight, and the screen is a pretty decent size. It's not for gaming, though, so I'm mentioning it because there are PC equivalents out there that might interest you...if you can find one with a dedicated graphics chip. Intel HD Graphics 4000 really isn't horrible, but Skyrim would bring it to
its knees.