As Rathji mentioned, don't get much more then a 15" if you plan on moving it around. I recently moved from a 13.3" to a 15" laptop and at times I miss the smaller form factor as I do lug it around on a near daily basis. But I do hook it up to an external monitor at home (as well as keyboard and mouse). The setup works perfectly for me.
I'm more versed in Macs (which Starcraft 2 will run on

), but there has to be a PC equivalent to the video card used on the Macbook Pro. I'm able to toggle it between low power usage on battery and high performance when I feel like gaming. That's a difference of nearly 2 hours battery life. So you get the benefits of mobility, but can switch it to be a decent gaming machine when you need to.