Probably 13 inches at this point. Been using a 13 inch Air for years now and its really the standard for me. I've gotten away with taking notes in class lectures via an iPad Air 2 w/ keyboard, but I wasn't bouncing around a bunch of apps and stuck to just having evernote pulled up.
I've tried using a Macbook for an extended period of time and I felt just a bit cramped for some things. As a primary computer, I couldn't get away with it. I have used a Dell XPS 15 with the small bezels and found it to be not overtly large like other 15 inch laptops (including the current Macbook Pro). I'd say that about the biggest I'd be able to go for my main computer. If the next gen 15 inch pro is akin to the XPS 15 in terms of size, I'd certainly consider it.