Overheard a person tell someone that commented on the size of his carry-on (Guy actually said that if it was the right size he wouldn'tt have to shove at it and dislodge other bags) that it was for him, wife and child so 1/3 each and that it didn't take up any more than 3 carry-ons.

And as Carol mentioned about the 2nd carry-on, they had a bag that barely fit under the seat, a pocketbook, a satchel the kid carried and a laptop.