WorldMonitor

What does WorldMonitor do?
WorldMonitor will check your worlds' sizes and notify you if they are taking up more disk space than they should be. The minimum disk size a world should be have to be considered oversize is configurable.

Full feature list:

• Well-commented configuration.
• Oversize world notifications for players and console/logs.
  • On server/plugin load.
  • On world load.
  • On player join (if they have permission).
• An overall world status command.
  • Displays all worlds and important information about them.
• A world-specific information command.
  • Displays very detailed information about the specified world.
  • WorldBorder support to display the world's border size.
• An efficient reload command.
• Display world sizes in IEC or SI units.
• Really lightweight.

Commands & Permissions
/worldmonitor overall - worldmonitor.overall (Defaults to Console/OP)
/worldmonitor info <World> - worldmonitor.world.info (Defaults to Console/OP)
/worldmonitor reload - worldmonitor.reload (Defaults to Console/OP)
worldmonitor.warn (Defaults to Console/OP)

Installation
Simply drop the JAR archive you downloaded in your server's plugin directory.