Trust me, CPU frequency will make a difference of max 2 fps in game. I tested it with my i7 3770K: running at 2,5 GHz or 4 GHz makes zero differences in games.
The best way is to find a laptop with a dual core hyperthreaded i5 and a GT 650m/GTX 660m/GT 750m/GTX 760m. This would be the best choiche.
It varies from game to game. It's largely dependent on how much physics computing the game does. In something like Skyrim, Red Faction, or Borderlands 2 and Batman Arkham City with PhysX effects turned on, it does matter. Less so with an Nvidia GPU with PhysX, but even then it only takes some of the physics processing load off of the CPU. You can get away with a less powerful CPU in physics heavy games with an Nvidia card, but you still need something halfway decent.
Still, you want a quad core CPU at least in a gaming PC regardless of whether it's a tower or a laptop. Regardless of how much the frequency matters, having a quad core does matter in a laptop or PC that has a primary function of playing games. Most games are optimized for quad core CPUs there are even a few newer games that require one. A dual core will do in most situations, but for optimal performance a quad core is important. Those extra threads make a difference and can be the difference between running certain settings on low or high.
What you don't need is a 6-8 core. No game uses more than 4 cores in a CPU. Having the extra cores can actually cause problems with some games. It can actually reduce the performance of a CPU to have too many cores because any more than four won't be used in a game regardless. For modern gaming a quad core is optimal as most new games are programed to take advantage of a quad core CPU's capabilities.
Frequency can be compensated for, but I advise against a dual core CPU for a gaming laptop, even if it's not 'high end'. Most games are made to take advantage of having one these days.
Also, the size of the screen is less important than the resolution. A 17.5" screen will perform the same as a 15" screen at the same resolution. The GPU doesn't work any harder because the screen is bigger. So either way you'll be fine. If framerate is an issue with a 1080p screen, you can lower the resolution in the settings to 1366x768 and it will run just as good as a screen with a native resolution of 1366x768. Don't worry about screen size and resolution if the price is good and it has all the other features you want. On a 15"-18" screen you likely won't notice the difference between 1080 and 768 anyway, and either way the resolution can be adjusted if you need to squeeze a few more frames out of a game.
The only thing a larger screen will impact is battery life, and that's a minimal impact at worst. It may not be an issue at all as a laptop with a larger screen is likely to have a larger battery as well.
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