I've used a 13" laptop as my main workhorse for 7 years now and can't imagine trading it for a bigger laptop.

The reason for choosing 13" is that I travel a lot for work and I want to get some work done when I'm on the road. I also find it the ideal size to still accommodate a decent-sized keyboard and trackpad. When I need some extra screen estate, I just hook it on an external monitor.

Because I do allot of programming on it, I've chosen one with a high resolution screen (1920x1080) so I don't trade in screen estate but that might be a bit too high for most people (e.g. everything tends to get a bit tiny).