Activate the tool button and digitize a polygon in the map window. Double-click to close the polygon. The determined area size and the scope of the object are displayed in the message window.
 
Activate the tool button and digitize a line in the map window. The determined line length is displayed in the message window.

The measurement is based on the projection and is usually more accurate than a measurement with on-board resources.

Tip: To change the number of digits after the decimal point, hold down the Ctrl key while clicking.