A donor heart valve sizer and method of sizing to increase the quality and yield of, and decrease the expense of fabricating, prosthetic heart valves. The sizer includes an axially-extending sizing portion sized to fit within the lumen of a donor heart valve. The sizing portion may be conical such that a resistance to further insertion is felt when the exterior of the sizing portion contacts the annulus of the valve. A measuring bracket attached to the sizing portion includes a scale aligned with the sizing portion that indicates the final expected valve size based on the position of the fresh donor valve on the sizing portion. The measuring bracket includes regions of acceptable valve sizes, and may also include regions of unacceptable valve sizes. A method of sizing includes utilizing the sizer in the slaughterhouse to reduce the overall number of valves shipped to the valve assembly facility to obtain a particular valve size. The sizer and method are particular useful for sizing xenograft valves, especially porcine aortic valves. The sizing portion may be made of polytetrafluoroethylene, and have an included taper of approximately 4°.