Tension Software (www.tensionsoftware.com) has revved Precise Screenshot, a tool for taking screenshots on your Mac, to version 1.1. It’s a maintenance upgrade.
Precise Screenshot can select a resizable portion of the screen with the usual drag method or it can use pre-fixed size area from a customizable popup menu (with the sizes the user needs) or also use the full screen area. You can specify a new specific screen size. To take screenshot with high precision is a matter of selecting it from the custom popup (or resizing and dragging it in the traditional way).
Precise Screenshot requires Mac OS X 10.9 Mavericks and costs US$4.99. It’s available at the Mac App Store.