[dragontamer]

17" gives enough room for a numpad (some 15" have it too but it's pretty universal at 17"). It seems to be where screen space is comfortable for long term use

But if I'm going into a small airplane (ex: Spirit Airlines), such a laptop is impossible to use.

It really depends on what your on-the-go space is. 15" seems like the overall standard to me however.

[dv_dt]

I respect the requirement some have for a numpad. However, at this point, I don't even have a numpad for my desktop workstation. I like the shorter reach between the home keys and a mouse on the right.

[cosmic_cheese]

Best of both worlds in my opinion is an external numpad, which can be placed to the left of your laptop/keyboard (which if you’re right-handed, also enables simulatanous numpad and mouse usage) and when it isn’t needed, tucked away.

[jcranmer]

I personally find that the limiting factor for a laptop on airplane is how much I have to angle the screen to be able to see it, which is how far forward the laptop has to be for the screen to go that far back. At that stage, the laptop is so far forward for me that it is uncomfortable for me to use the keyboard.