Update: DS now used 72 dpi for size reference. This is unfortunately not an appropriate dpi for print, so should only be used for true size applications.
You may recall that in DS2, all bitmap images imported at 2.4 inches high, regardless of their original pixel count or resolution. This meant unless your images started at 2.4 inches they were always incorrectly sized, both absolutely and relatively. An unannounced feature of DS3 is that images set to 144 dpi resolution will import at correct size. So, for instance, if you scan a stamped image with your scanner set to 144 dpi (unfortunately this is not a standard setting so not always a direct option), then when you import the scan to DS3, it will stay the actual size of the original stamped image.
This also means that a set of images of the same resolution will import at the correct relative size now. 144 dpi should work fine to make a cut only file, but it’s too low for print images. So my advice if you are scanning an image for print then cut is to save at 288 and then scale the image to 50% (divide one dimension by 2 with the proportion lock on) after you import.
If your scanner does not let you specify 144 (or 288) as an option, scan at 300 dpi and use an online DPI converter to change it to 144. Or if using Preview, Photoshop, etc, change the DPI *without* resampling.
Update: also see this video for a simple sizing method using a reference shape.