Quick Tip: Making precise-sized selections, method #1
If you know the exact size you want to make a selection, there are a couple of things you can do to get there. The quickest and easiest is to switch to the Rectangular Marquee tool, open the Info palette (shortcut: press F8), and as you start dragging your selection, glance in the bottom-right corner of the Info palette and you'll see a W (for Width) and an H (for Height) reading. As you drag, you'll see (in real time) the size of your selection, displayed in your current unit of measurement (inches, pixels, or whatever you have it set to).
The inspiration for this one came from a zoo T-shirt I saw someone wearing. You start with photographs and convert them ...