Select Closed polygon from the Measurements drop down menu (or press the G key).
Click left mouse button to deposit the starting node. Repeat this process to deposit the remaining nodes around the structure you want to measure.
You will be able to edit the final shape afterward if need be. Double-click or click the starting node to finish drawing.
The values measured are area (cm2) and perimeter (cm).
You can read more about the Closed polygon tool in the 2D viewer section.