Navigate to <https://afterglow.skynet.unc.edu/en/workbench?tool=display>

Open your file.

Make sure you can see the face of your planet without much glow around it.

Click the icon for taking a measurement. I call it the "line" icon because I need to "draw a line" to take the measurement.

Click once on one side of the planet. 

Let go of the mouse button.

Click on the opposite side of the planet.

Let go of the mouse button.

Scroll down a bit in the measurement panel that is already open.

Copy and paste the measurement you need, usually arcsecs. Copy the number only, not the unit.

If you are measuring Saturn, measure across the face of the planet, not across the rings.

If you are measuring a planet that is showing a phase, measure across the widest part of the planet.

Afterglow: Measure the Angular Diameter