Get the largest screened, fastest processor, most ram you can get.
DO NOT go smaller than 15" screen. After that, try and max out your RAM with at least 4Gigs.
Honestly, for most everything you do, any of them will WORK, but the faster, bigger you get is going to make your life SO much easier. Faster you go saves so so so much more time after a while, and you will find that if you are in any sort of an intense program (and serious about it at all), every minute your computer doesn't slow you down is going to be very very valuable.
I'm trying to crank out files that end up exceeding 1.5 and 2 gigs on a macbook pro, older version, with 4gigs of RAM. It's not fun. Even with smaller files - multiple page InDesign or photo processing workflow my computer can't keep up with me, and I waste sooo much damn time on it. Your workflow and how you set up files can help, but only so much.
So sparknotes - go big or go home.