If you ever find yourself needing to measure your screen, not in inches, centimeters, or other conventional measurements, but rather, in pixels, then follow this guide for a short interlude on a built-in OS X function that can aid you tremendously.
There is a built in screenshot keyboard shortcut, CMD-Shift-3, that can take full screen shots, but there is also a CMD-Shift-4 that allows you to define the size of the shot. This is the tool we are looking for. With it, you can drag a box around the object you are measuring. However, when you get to the end of the box, do not let go to take the shot, but rather hold it so that you can view the pixel sizes.
There are also a few other apps that can help you in the process, however, for something that is quick and easy, the built in screenshot command is just what we need.
If you have any questions or concerns, post them in the comments below and I will get back to you as soon as possible!