Reported length measurements from the Shroud have been done for over a century. Unfortunately, almost all those measurements are not reproducible as they do not report their endpoints. This online tool provides a simple but effective means to do so. Essentially, a reported measurement includes its endpoints as rectangular coordinates (a.k.a. points).
This method is useful since reporting a length of an "image body part"—without its endpoints—cannot be objectively verified: almost all such images have no definite borders. For example, one can report a face width of 15cm for the Man of the Shroud, but it is quite unclear where that measurement was done. It is very difficult to verify these basic claims without further objective data. At the very least, the endpoints of these measurements should be provided.
The online tool reports the length in pixels and millimeters. The first value (i.e. in pixels) is directly taken from the image, but the second (i.e. millimeters) is based on a calibration of the photograph. This calibration is different for each photograph listed below. You can read the details of the calibration of the restoration photograph (2002) here—but you should skip that if you just want to do some length measurements.
Length measurements can be done on three photographs: