With the right backpack, traveling with a seventeen incher isn't that bad. But it's still less portable than a 15" laptop.
When I first got my i9400, I thought it was huge. Then, months later, getting back my old 15" from the repair shop, I couldn't help but find it incredibly tiny.
I'd say 17" is about as big as you can get and still be portable, and even then while the i9400 is relatively portable, many larger 17" notebooks from companies like Sager or Voodoo wouldn't be.
As an interesting side note, the i9400/E1705's 17" screen is the exact same width as a desktop 19" 4:3 LCD screen. So for widescreen video content, you're effectively watching it on the equivalent of a 19" LCD
Of course, I just got my 37" LCD HDTV, so I won't be watching too much on the laptop's screen for a while