To make your popup responsive you should do the following:
Select the "Custom mode" in the Dimensions section of your popup settings.
Set "Width" and "Height" in pixels (px) and "Max width" and "Max height" in percentages (%).
When the screen is too wide, "Max width" option will work and it will prevent showing a too large popup.
Max width: 80%
Max height: 90%
Min width: 300 (px)
Min height: 100 (px)