Gordon – I've got a 13" MacBook Air. For most things it's fine. But there are times when I want more. So I also have an external monitor that I use much of the time when I'm at home (with the Mac running in clamshell mode). I used to use a 24" external. My early Christmas-birthday present was a 27" monitor (Dell) with a 2560 x 1440 resolution – the max my Air will support. Great screen. Four downstream USB 3 ports (that could have used more power).
While it might be nice to have a 15" laptop, I'd end up with a machine that was heavier = less portable, and it would have meant spending more and paying for more power than I really need. What I saved in the cost of the laptop was spent on the monitor, but the monitor ought to outlast this laptop and carry me to the next.
Consider whether you want to carry around a larger machine. It may be that going with laptop plus monitor would work well for you, too.
Here's a shot of the workspace I use everyday for my Bible reading.
Screen Shot 2014-11-27 at 10.25.46 AM.jpg 161.54KB
I think I set it up when the laptop was connected to the external display.
MacBook Air Yosemite
mid-2013 1.7 GHz Intel Core i7 (2 cores)
8 GB RAM; 512 G SSD
Mac mini Yosemite Acc 11
late-2012 2.3GHz Intel Core i7 (4 cores)
16 GB RAM; 1.12 TB Fusion Drive
iPad Air 1 iOS 8.1