Large, a bit buy not heavy
Occupational Therapy principles state that the total weight of the bag is within 10-15%, the person should not suffer shoulder strain
If a person weight is 80 Kilos, they should be more than ok with a 8 Kg bag
I know a mature lady who uses 15 MBPr without any problems and she travels a lot
But its is also about perception of each one and I appreciate that some people likes less weight
But this does not mean that the perceived real weight would be harmful
A reasonable healthy individual may have health issues if they are unable to carry between 10-15% of their weight
I put the laptop inside my bag, in my car and I only carry the bag from car park to the office and viceversa
Occasionally, I flight and I use iPad, read book or even use 15 MBPr without any problems. I will buy MB 12 inches as a secondary machine as it is very nice, but not essential in my case.