You can crop an image using another image's dimensions as the measurements for the crop. To do so, open both images, then go to the image whose dimensions you want to copy, and go into the crop size menu and choose Front Image. Then go to the second document, and you'll see the crop tool active with the dimensions of the first image.
In this Adobe Lightroom free tutorial, you'll learn the intermediate photo editing tools. Improve your pictures with crop, HSL, curves, sharpen, vignette and much more! Plus, this lesson links to all the others in this Lightroom photo editing series. Click thru to learn!