Crop to a Specific Size?

Click on the Crop Overlay tool (R) in the toolbar beneath the Histogram panel (in the Develop module’s right side Panels area), and you’ll see a bunch of tool options appear. There’s an Aspect pop-up menu there and by default it’s set to Original. Click on that pop-up menu and a list of preset sizes and ratios appears. Just click on the one you want and the cropping border instantly changes to reflect that aspect ratio.