I'm guessing that screen estate is important so 17 inch should do it.
Just curious what everyone is using and where they got good deals from.
I've found it's awesome because I can effectively go from web browsing on a 1440 x 900 laptop to working on the equivalent of an iMac (2560 x 1440) or even higher (2880 x 1800) without losing sharpness.
If you then want extra space, you can plugin in an external monitor, but to be honest, I ended up selling the two externals I used to use (they were 1080p, and seemed small and blurry in comparison).
Using OSX, you also get the benefits that it's very unixy/linuxy, so you get bash, ssh, openssl, grep, sed, ruby, python, php, apache etc as part of the system. With Xcode you get other tools like git. Configuring an acceptable dev environment for web dev can be done without downloading anything extra. (Of course, to get an awesome dev environment does require a bit more work, but arguably much less than the equivalent on Windows).
I Bought it "used" for $500. (i3, 500gb ssd, 4gb ram)
Personally, I think the most ideal web development machine is the 13 inch Dell XPS Developer Edition with Ubuntu.
Some of the best things about it are the built in 3G modem with 2 years free internet, how light it is, how cheap it was even though it really looks very nice, and how quickly it boots.