What's the difference in portability between an 11" and 13" screen? To me 11" or 13" wouldn't make a difference but whether it was 2 lbs. or 4 lbs. would be a bigger factor. But that's just me. Thinking about this generated the following rough breakdowns of size, weight and battery life.
Luggage size: 17" and bigger
magazine size: Around 9" or 10" and larger
Jacket pocket size (like a checkbook) about 6" or 8" diagonal
Phone size: less than 6" diagonal
Weight
More than 7 pounds: wheels required
4 lbs. or more, carryable in a shoulder bag or backpack.
2 to 4 lbs. Painless to carry
less than 2 lbs. you don't know it's there
Batter life
Under 2 hours: add the weight and volume of the charger to the specs
2 to 4 hours: Probably enough to get you through the day or on a cross country flight. Most of us don't use the computer constantly. We also eat, talk to other people and such so that the computer is not on all the time.
Better than 4 hours, amazing.
You can figure it out. For some ultraportable means an iPhone. For others, size doesn't matter as much as weight and for others batter life is the limiting factor.