My daughter is flying for the first time next month and we drive to Disney so this is all new to me. My question...I have the carry on luggage information and I know she can have a second bag but I can't seem to find out how big that bag can be. Someone told me it must be able to fit under the seat, but how big can that be? And does that second back include her purse or can that purse fit into that second bag? Sorry for all the questions and thanks for your help.