I received a 14" laptop for use as part of my grad program. The battery will last at least 4 hours doing moderate work. I can use it an an airplane without being cramped, and just about anywhere else. It has enough hardware to process any data set I may be working on, and is faster than my home desktop which I just built maybe 6 months ago albeit on a budget.
When I'm in the lab or at home, I just plug in another monitor, and I'm good to go for doing real work.
14" to 17" to 19" doesn't buy you a whole lot in monitor size, especially if you are using some kind of desktop management software that will split it into multiple desktops. Another monitor is best.
At home I have 27" connected to my desktop, and split it into multiple windows, but I still like the dual monitor setup better.
I think if you are going to have a laptop that gets moved around a lot, this is about the perfect size, and I love it.
I wouldn't mind having a bigger one if I had to transport it less frequently, I move mine up to 4 times a day.