I'm a full-stack web consultant (Rails/AngularJS/React).
Started this journey w/ an 11" MBA 3 years ago to take on my first Rails project. My previous setup had me working on .NET software w/ 3 displays (2 24" and 1 21") and I wanted to see if I could survive with much less.
After being with the 11" for a year, took a job for another year where they insisted on a maxxed a 15" rMBP. I dug the machine... fast, tons of screen real estate at the max res, but to be honest, it was just ergonomically lacking in too many situations (would get hot on the bottom, somewhat unwieldy in my lap, difficult to carry in one hand to show someone something). Plus while the battery life wasn't terrible, it would routinely last ~6 hrs, so not really an all day machine without outlets in the vicinity.
Now I'm back to being a consultant I decided to go w/ a 13" - I literally had a new 15" in the box ready to go, but decided to test out Apple's liberal return policy on a 13" as an experiment. It hits that sweet spot for me. Rendering on the screen is a bit laggy at times, but not in any way that impacts my work. The loss of screen real estate is just a minor issue - I didn't really *need* 1920 x 1200. The overall speed of the machine is hardly perceptible for what I do. And the battery life... seems on average I'm probably pulling in an extra couple hours, which is huge. I can go without charger for an entire day, work in situations without a table to put it on for an extended period of time, and have it fit much better on a cramped cafe table. It's a far superior experience for me and there's no way I'd go back... not sure how I'd feel if I were doing graphics work though.