I've set a couple of raster images over CADD files for background/digitizing purposes, and have not yet found any easy way to do it. If I don't have the exact size of the image, I simply attach it. Now, in order for this to work, you have to have an object on the CADD file that corresponds to an object on the raster file.
Using the Raster Move command, line up one corner of the objects and use the Active Scale Distance command to set your active scale to the difference between them.
Draw a line between opposite corners of you raster image (line them up by eye) and scale the line with your active scale. Then use Raster Modify to scale the raster to the line (by modifying the corner of the raster to the end of the scaled-up line).
Go back and Raster Move the image to line up again with the object on CADD. Iterate until you are satisfied with the fit.
If anyone knows an easier or more precise way of doing this, please let me know.
Rob