Use The Crop Tool’s “Front Image” Option

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.

Front Image

You can also watch this video on the Crop Tool’s “Front Image” feature.