Its not too big. As long as you have something decent to carry it with I don't see why you should worry about its weigth (it weighs a lot less than my old 14" dell inspiron). At 1" thick it is the slimmest powerbook.
I would be concerned about the battery though... While I feel it lasts enough for my needs, the 17" IS the one with the worst battery life... The screen just uses way too much power. You can get about 4 hours of word processing use (for example) if you turn down the screen's brightness to the minimum. However, I think the battery life is pretty decent if you consider the size of the screen.
Anyway the best way to decide is to go to the store and check out all three of them and carry them around for a bit. I do feel the 17" is the coolest, and once you get used to it, you will realize that the other powerbooks are just TOO SMALL...