This is the way I get the true size of scanned images.
On the the drawing, choose two pairs of points that are easily found on the imported image. One pair in the X-direction and the other in the Y-direction.
Measure these distances as accurately as you can - let's call them xd and yd.
In model space, use the dimension tool to create two dimensions; one between the two x scale points and one between the two y scale points. Suppose these turn out to be xm and ym.
Select the whole image in model space, including the the two dimensions, and then tab into the x-scale field, press F2 to bring up the calculator and type xd/xm and press enter. Copy and paste the result into the x-scale field, hit tab to select the y-scale field, use the calculator to compute yd/ym and copy/paste the result in the y-scale field. The press Enter to scale the entire model correctly. The dimensions should now show the measured values and the whole image is the correct size.
I do the x and y scales separately because scanners often generate slightly inconsistent aspect ratios. If you are satisfied that your scanned image has the correct aspect ratio you can use a single dimension and insert the same ratio in both the x and y scale fields.
TC21.2 x64 Plat, Bld59
TC16.2 Plat, Bld54.0