If you have a door image for instance and know the exact dimensions of that door,
make a simple rectangle of the exact size in SU;
import the image as material (see this option at the bottom right of the import window)
apply it to the face you've prepared by scaling it while applying to fit the whole face
and voilá; you're done. Your texture will appear with the correct dimensions under the "In model" tab in your material browser and you can simply drag and drop it into any collection in your secondary window for later use.