Crop to a Specific Size?

At the left end of the Options Bar, click-and-hold on the preset pop-up menu and a bunch of cropping ratios and sizes will appear. To select a particular crop ratio or size, just choose it from that pop-up menu (like 4 : 5 [8 : 10], for example), and now when you click-and-drag with the Crop tool (C), it’s constrained to that 4 to 5 ratio size. If you want to create your own custom ratio or size, just type it in the two fields that appear directly to the right of that pop-up menu (and to quickly clear those figures out, click the Clear button to the right of those fields). By the way, those two little arrows in between the custom ratio fields swaps the two figures (so, clicking on it makes a 5:7 ratio 7:5). If you change your mind and want to reset your crop, hit the Esc key on your keyboard. To cancel cropping altogether, hit the Delete (PC: Backspace) key.

TIP: MORE CROPPING OPTIONS

Once you have the Crop tool, if you Right-click anywhere inside the cropping border, you’ll get a pop-up menu of all sorts of things you can do to your cropping border, including using different preset sizes and resolutions, resetting the crop, etc.