To make a document the same size as another open document, go to the Image Size or Canvas Size dialog boxes (Image > Image Size or Canvas Size).
Then, while the dialog box is open, go to Window and select any other open document, which will be displayed at the bottom of the Window menu. Photoshop will automatically fill in the values for that image.
Press OK, and the image will re-size.
You can also try the “Front Image” feature to do something similar with the Crop Tool.