Seems the default icon is xfce-mount.svg.

But when i set panel preferences to use 128 pixels row size the icon seems to only go up to 64 and then it just adds margin from 64 to 128 instead of making the icon bigger.

So i suggest making it scale above 64 pixels to so it uses all the space and looks consistent with other icons.
Not that all the other plugins icons looks good but we can fix them one at a time until they are consistent.

Version 1.1.3.