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, 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