I have a 17 inch laptop as my primary computer, the size is somewhat unwieldy. I'm not sure I'd necessarily go smaller but it is something to consider. I'm also not sure how much traveling your going to do or to where but a Toughbook seems to me to be a bit overkill for average traveling. Something like a MacBook Pro is extremely solid but not as solid as a Toughbook.
To answer your question, it looks like Acer makes some 17" laptops with magnesium casing in their
TravelMate series but I failed to find a site selling it though. From what it looks like, all the extremely durable laptops are 15.4" or less.