Just to give you an idea, my screen is a 15.4 inches (widescreen). I work on this laptop every day and it's big enough for me. I wouldn't want to go smaller though. Yes, 17 inches would be nicer that's true... but there's no way I could carry it.
I guess the best is to get a light computer with a small screen for portability, but have a docking station where you can plug an external monitor, as big as you like, if you regularly work at the same desk. But it's not an option for everyone, that's for sure.