I made a Macro for easy scaling of images in Image workbench. It works for images imported as planar image in the 3D space.
You need two points in the image that you know the distance between for the real object:
* run the Macro - a dialogue pops up
* click on the two points in the image that you know the true distance between
* Select the Image Plane in the tree view
* Enter the true distance in mm between the points in the text field of the dialogue and click OK
*The image is scaled and the dialogue closes.
(I have tried to make this version more user guiding and mistake safe than the earlier version I mentioned in an another forum tread)