Custom Icon Controls in Feature Widget

I am trying to add a custom icon using the Features widget but having troubles sizing the icon so that it fits the container neatly.
If I understood this correctly, I upload my icon to the media library and select it using the Icon Image settings. Now, which image size do I select in the control below, and also which image size do I use in the icon size control at the bottom of the page? Nothing I’m doing seems to make a difference when I preview, and the icon is too large for the circle in which it’s supposed to fit. This is a great feature by the way, if only I could get it work properly!