Free, no advertising and no need of permissions to install!
You can move the slider to get a better measure. The "laser" that comes from the slider will tell you the exact length in centimeters. Just put the small thing on the screen, with one end at the 0cm position and move the slider to the other end. Be careful with the things you put on top of your screen, it could be damaged with hard and heavy things, I will not be responsible for the damages caused to the screen by the use of Ruler cm.