You should use the worldedit cui mod if you can, this is a mostly for fun project.

Requires skript-reflect
You must put a copy of the world edit plugin jar in the skript-reflect folder, too.

This skript will display the boundaries of your world edit selection. If the selection is not cubic, 2d poly, or sphere/ellipsoid, it'll show the cubic bounding box instead, which will not be accurate to the blocks that are selected.

Use //cui to toggle the visualizer on and off.

Default permission to use the command is server.worldedit.cui