A sterile, invasive instrument intended to be used during open vascular surgery to approximate the size of a blood vessel for selection of an appropriately sized vascular device/implant (e.g., vascular graft, emboli filter); it is not intended to obtain a precise measurement. The device typically consists of a C-shaped (half circle) sizing portion with a handle, sometimes available in a range of sizes, and is intended to size the outer diameter of the blood vessel (i.e., extraluminal sizing); it may in addition be used to size a vascular graft. This is a single-use device.