Use Enlarge, Calc Distance, pick to points of a known distance, then select your image.
If your image width to height ratio is off, you can use Datacad to measure and figure out how much the one direction is off and then use an image editor to stretch/enlarge in one direction to get things back to scale for that direction.
Thanks! - Josh Do. Or do not. There is no try. Josh's Digital Downloads is come back online soon. Stay tuned. (soon is a relative term)
I find that manipulating the bitmap image files as a a self xref or even an external reference file, offers more flexibility and control than trying to work with it directly. This is especially helpful when trying to rotate the bitmap but works very well for enlargements and even cropping.
James Goodman, AIA Capistrano Beach, CA Since V2.5f (1985)