If you really need screen estate, get an external monitor. I've used a 15 inch MBP since 2008 and before today I couldn't have imagined downgrading. I bought a used 11 inch MBA off gumtree today and have surprisingly found it more than adequate for the job. In particular I'm enjoying the prospect of truly mobile computing.

Personally, I think the most ideal web development machine is the 13 inch Dell XPS Developer Edition with Ubuntu.