Suggestions...
As a previous owner of both a 15" and 17" powerbook (and the current owner of a 12"), I must say it COMPLETELY depends on what it is that you do.
17"
Yes, it is kinda heavy (still lighter than most PC laptops, especially at that size) and tough to open on planes. Its battery life is a little lacking.
HOWEVER...man, the power, speed and screen size of this thing were great. It was AWESOME to open this puppy at a coffee shop or in class and get the envious stares from people around me. It was also cool to know that I basically had a desktop replacement that I could take from place to place.
Recommendation: Unless you do a ton of travelling (class, etc), this is a great machine.
15"
Lighter, but still noticeable in my backup. Nearly ALL the pros of the 17" (minus the screen, of course) and what I think would be perfect for those needing a little extra power but also require portability. Still kinda big to open on an airplane.
12"
The ULTIMATE traveller's notebook. Smaller and lighter than most of my books, better battery life (the specs may not say that, but I can say from real world experience it's about 15-25% than the other two).
No, the screen is not as big, but it's just as nice and sharp. I LOVE being able to carry this thing one handed (the others were a little too big for me to trust myself).
It's fast enough for all that I do (photoshop, FCE, word, keynote) but when I play Halo I do long for my old 15".
I will eventually buy a 20" ACD to go along with it. Price wise, it's about the same as a high end 15" and still less than a 17".
My recommendations? Unless you have ONE place you do most of your work (I just got a job at a magazine, so I leave my monitor there), I would get the 15". If you want something else, it's really easy to trade down (I sold my 15" to my sister, who doesn't carry it around as much as I did).
NONE of the powerbooks have such huge drawbacks, I never would have traded any of them for the other while I had them.
Note...my 17" got stolen and I couldn't afford another, so I got the 15". I owed my sister a ton of cash, so I gave her the 15" as payment and got a 12".