To activate the Ruler tool, click the 
button or use the Alt + L hot key.
The tool allows you to measure the length of any object, and change the scale of the scene.
To measure the length of an object, press the LMB at the starting point of the object, move the mouse pointer and then press the LMB at the end point of the object.
As a result, a red line appears on the scene. The length of the line is equal to the length of the measured object. The length of the current line is displayed in the Length field. The used measure unit is displayed in the Measure unit section.
To change the scale of the scene, do the following:
- Measure an object of the known length, as described above.
- In the Unit section, select the desired measure unit.
- In the Length field, specify the required length of the object.
- Click the Apply button.
As a result, the sizes of all objects on the scene (except for people) are changed in accordance with the new scale.