There's little difference in the gaming experience. Let's assume both resolutions, processors, graphics cards, everything... are the same... in a 15.6 inch screen laptop and a 17.3 inch screen laptop. The difference then, is ONLY screen size. And that too, only a 1.7 inch difference, which isn't really much.
As mentioned above, the 15.6 will be a lighter laptop and easier to carry around. If you're considering getting a new laptop, don't worry so much about whether it's 15.6 or 17+ inches... look at the processor, RAM and GPU... Assuming, of course, that the laptop has a 1920x1080 res.