look most of the games will not support 15.4" I am playing 15.4" and only tomg raider supports the widescreen look.So, you might as well go for 13.3 if you are getting additional HDware with it.
13.3" is more portable over the performance factors. They are made for portability thus the screen is smaller. But the problem with these are most of the times they does not have a modestly powerful hardware to run the games in moderate settings. 15.4" is the standard size for the laptop which has the balance of both performance and portability.
13.3 and 15.4 any you buy would have almost same weight, your carrying arround wont be affected by this. Take 15.4 screen size because 13.3 becomes too small and even 1280x800 resoultion would seem to be bad on it. only if you wanna strain your eyes alot.
15.4 comes in lappy category very well its not 17 inch that it will look very big.
Guys portabilty comes with weight of the lappy and not just size(not for 13.3 or 15.4),
infact 13.3 and 17 both costs more than 15.4