Edit: If you want to keep the original dimensions (as in mm) please don’t copy/paste from images of different resolutions but place them as Smart Objects (and make sure they place at 100% which is indicated in the Options Bar).
Edit: If you want to keep the original dimensions (as in mm) please don’t copy/paste from images of different resolutions but place them as Smart Objects (and make sure they place at 100% which is indicated in the Options Bar).