Abstract: Medical devices and methods for determining the size of blood vessels are disclosed. In an embodiment, a blood vessel sizing device includes a marker configured for placement on the skin of a patient. The marker defines a substantially circular shape and includes a plurality of radiopaque substantially concentric circles. In an embodiment, a blood vessel sizing method includes placing a marker having a plurality of substantially concentric circles on the skin of a patient, imaging a blood vessel of the patient and the marker, and comparing the imaged blood vessel to the imaged circles to determine the blood vessel size.