First of all, a 13" laptop will be annoying to program on. That size screen might end up being too small. More screen real estate is better, it will be much easier to get things done.
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If I were you I'd go 15". The integrated chipset should be fine, you have a lot to learn before you get into higher-end graphics.
In principle I agree, however, screen size != screen real-estate. There are plenty of 15.6" laptops out there with the same 1366x768 resolution you'll find on many 12"-13.3" notebooks -- even some as as small as 10.1".
You want a combination of resolution AND screen size to provide enough pixels which are large enough to not strain your eyes. You can find a few 13.3" notebooks in the range of 1440x900 or 1600x900 -- notably the 13" macBook Air, and there's a Sony Vaio that I'm aware of. The MacBook Air is probably out due to rather old components inside currently -- however its rumored that a whole new wave of mac products will be launched alongside OSX 10.7 very soon, including new Airs, Minis, and MacPros.
On the PC side, its worth noting that AMD's new laptop platform, based on the Llano Fusion APU will be on shelves within the month, possibly weeks -- early reviews are promising -- with CPU power competitive with lower-end Intel CPUs and GPU power comparable to low-medium-end desktop GPUs (400 shader cores, plays many modern games at native resolution at playable framerates). The best part is that you can expect to see these machines appear for $600-$800 probably, and with battery life topping 8 hours.
If its not a pressing matter, I would recommend waiting a month or so to see what's new -- normally you can't play that game with PC hardware because it will always be "one more month", but in this case the market shift is going to be pretty dramatic, and the schedule is known.