In my opinion, notebooks are supposed to be portable and 17" is pushing it. I personally wouldn't go above 15" and just get an external monitor if needed.
You mean 17" is the limit, because there have been PC notebooks who push it and it is just ridiculous to say the least, because

The 17" MBP is simply perfect and a true marvel, an awesome marriage of sleek, professional, portable and screen size. It is the original MBP and will always be.
The machines Apple are now pushing out like hot cakes, are just that, with lots of frosting and syrup.
The 17" MBP is in no means "ultra portable" but it remains very portable for what it is, compared to the other 17" notebooks out there because most of those are strictly gaming units which stays anchored at home just like the bottoms of the gamers.
Since late 2006 I have been carrying my 17" MBP everywhere, traveling, touring, gigging, field recording, video making, love making etc etc etc, it is everywhere - one of the best tools of the trade for me. She is still working hard for me this early 2007 17" MBP.
These machines are already thin enough since the unibody design, the retina macbook design is the thinnest it can go before it is just so ridiculous.
If you want to talk about portable, I would go straight to the eyefone and just use it and keep my 17" at home, you know what I mean? What doesn't the eyephone do now a days?
But I hate fones, I like free universal wifi for all humans.
Thank you.