A surgical instrument for sizing the lumen of an organ is provided. The lumen sizer comprises a handle member with a projection at either end. Each projection has a substantially cylindrical sizing portion of different diameter, each diameter corresponding to the working range of a surgical instrument to be used on the organ or to the diameter of a prosthetic device to be attached to the organ. Tapered insertion regions having atraumatic tips extend from each of the substantially cylindrical sizing portions. The surgical instrument provides a fast and simple technique for measuring the lumen of an organ.