When using the Cropping tool sometimes it helps to be able to set the ratio to match a photo to a given print size. Additionally you can also set the required resolution for the crop to really speed workflow.

1. Open a photo file and 'Save As' under a new name to preserve the original.

2. Click the Crop tool and the lower menu bar will change to display the contextual settings. To check that the units are set to inches/centimetres go to Edit>Preferences>Units & Rulers to bring up the preferences menu, then make the necessary changes as shown.

Now when you drag the cropping selection box, the ratio you set will be retained. Just bear in mind that the crop will also resample the image when resizing so, depending on the size of your original file, how small you make the crop may affect the eventual quality. If the image is resized up too far it may produce artifacts.