A functional sizer and method of sizing that overcome the problems of relying purely on visual assessment of size in selecting an implantable heart valve device (repair implant or a heart valve replacement prosthesis) and of being unable to test the fit and performance of the heart valve implantable device until after it has been stitched in position. The invention provides a functional sizer that is functional and can be reversibly secured in position, largely using the valve sutures under tension, thereby enabling assessment of fit and performance prior to permanently stitching a heart valve prosthesis.
