Almost back pack size so can get to easily from under the seat in front of you, I carry is as a purse or put mine in it so I only have 2 total to keep up with, get liquids to drink after in the gate area, any liquids get checked shampoo etc. anything else in the separate small zippie.
Have seen a person try to sneak a small dog on in carry on, YEP she got to go to the double check side, and had some looks from security, told to get puppy checked by a vet asap. one of those WOW moments !!