Make measurements on annotations:

1. Select in an image the segments of annotations and regions of interest that you want to measure. Multiple segments and contiguous segments can be selected by clicking on the left mouse button while holding the 'Shift' key. You can unselect segments by clicking on it while holding the 'Ctrl' key.
2. Click on the "2D Actions" button of the 2D Viewer window. A menu will appear under your mouse cursor.
3. Click on "Measure".
4. The measurements have been created.

You can also make a measurement by 2 others methods: by the top menu of the 2D viewer window, or by the context menu.
See "How to launch actions".