Each carrier has their own guidelines when it comes to baggage restrictions (i.e. fees, maximum number of bags, carry-on restrictions). The universally accepted carry-on luggage size for US domestic flights is 22" x 14" x 9".
Many airlines allow for one carry-on item and one personal item small enough to fit under the seat in front of them. Passengers in bulkhead seating or exit rows may be unable to bring a personal item on-board. Please ensure you work with your Millennial Travel Group consultant to determine your seating options.