You can do online length measurements directly on the digital photograph (copyrighted Arcidiocesi di Torino). Its calibration is 1px = 1.985mm.

On the right, the current coordinates $(x,y)$ should be displayed as you move the pointer mouse over the image; with the length in millimeters from the last selected point. Every time you click on the image, new coordinates are selected and a length is calculated from the previous coordinates. (You must have Javascript enabled in your browser.)

Unknown coordinates until you click on the image. Unknown coordinates until you click on the image.