Framator is the perfect tool for people who need to make screenshots of a precise size, for instance, developers submitting to the AppStore.
It shows an adjustable frame on top of your apps which will
help you resize the windows or content you wish to screenshot to fit a precise size.
It comes with the preset sizes for the AppStore requirements for screenshots and provides a screenshot button to capture the content of the frame.
If you prefer OS X screenshot features, you can make your screenshot while the frame is in place, or disregard the frame and take your screenshot once the content has been placed and/or resized.