When a scope specifications are 3-9x50, the 50 refers to the diameter of the glass (50mm in this instance).
You would need to go to the manufacturers website to see the specifications on the eye piece.
Here is an example for a Leupold scope. Look at the "Dimensions" tab for an outline drawing.
https://www.leupold.com/scopes/compa...5x56mm-cds-zl2