To make your popup responsive you should do the following:
Set width and height in percentages (%) and then set Max width and Max height in pixels (px).
When the screen is too wide, Max width option will work and it will prevent showing a too large popup.
Max width: 640px;
Max height: 480px;
But, in case this option doesn't work for you anyhow, we have an option "Enable Scaling" in the Advanced options of our Pro version. This option will make your popup automatically fit the screen.