Which version of Vision Development Module do you own?
If you have the latest version (2010), I would recommend using the new Contour Analysis functionality. (Machine Vision>>Contour Analysis palette)
Open your image, use IMAQ Extract Contour to extract the longest contour, then IMAQ Get Contour Info will return the length of the contour in both pixels and real world values if you calibrated the image. In your case, you'll have to divide the value by 3 to as the contour is a closed contour containing both side of the arc.
If you don't have the latest version, this is still possible to do. Call IMAQ Extract Curves (Image Processing>>Analysis palette). From the Curves returned, I would suggest getting the one with the longest length. Divide it by 2 and you'll get your length in pixels. If you calibrate the image and want the result in real world units, then, get the points of the longest curve, call IMAQ Convert Pixel to RealWorld (Calibration palette), then IMAQ Point Distances (Machine Vision>>Analytic Geometry) to get the distance between the real-world points. Divide by 2 again and you'll get your length.
Let me know if you need a quick example with either method.
Best regards,
-Christophe
