When making a measurement using the polygon or length tool, you will drop a marker each time you click. If you make a mistake, you can click on the 'backspace' OR 'delete' key on your keyboard, and go back one click instead of having to redo the measurement.

You can zoom in and out of your project page by holding down the 'ctrl' key on your keyboard and using the scroll wheel on your mouse. Alternatively, you can pinch to zoom if you're using a trackpad, similar to a Maps app on your smartphone.