Created attachment 4471 
xfce4-diskperf-plugin-border-size.patch

Use better border sizes dependent on the panel size.

Use 0px on very small panels, which look ugly anyway and where
the only concern is to preserve as much space as possible.

Use 1px on small panels to optimize for the use of space, while
still trying to preserve a good look.

Use 2px for bigger panel sizes. Looks pretty and does not waste
space unnecessarily.