Here are few helpful links that describe how to insert an image, pdf, excel, etc into AutoCAD Model space and properly scale it to the right size.
- Type SCALE into the command line and press Enter
- Alternatively you can find this command on the Modify Panel on the Home Tab in the Ribbon
- Select the image you want to scale and press Enter
- Click at the base point
- Press R for reference
- Click at the first point of the known dimension (same as the base point), then select the end point of said dimension
- Enter the "true length" of said dimension and press Enter