The Utility Buttondown is designed to be over-sized. It should fit loosely, and it's fit is not comparable to other button-down shirts. If you are stuck in between two sizes, the way we usually determine the right fit is by the desire of the customer, do you want it to be more over-sized, or more fitted, do you want it to be longer or shorter. Do not be alarmed if your bust and hip measurements are significantly lower than the measurements of the shirt - it is meant to be this way and is necessary for an over-sized fit.
*As usual, all sizing is approximate and can vary slightly between garment*