I got this
Backpack + Dolly (purple) for $25 (now it's $30) for United economy without knowing the actual size (it's not listed on the details).
But it turns out that the bag basically maximizes the space available without going over. The dimensions are 15x8.5x10.5(easily crushable).
The dolly should help with shlepping things in the airport. However, as it is 10.5 wide and solid, I don't know if it will work when it is in or strapped to the bag, for the personal item. Maybe strapping it to the carseat will be better.
Does anyone know how makpid they are about the 10" vs 10.5?