I have created a simple php page that will list all the users and the total size of the media files they have uploaded. Hopefully this will be helpful to someone.
On Xibo directory create a file called myUserQuota.php and paste the script from here (https://gist.github.com/bickyz/1c5d26874d0b87d0511cc58f0e595002), then launch it via browser http://yourxiboserver/xibo/myUserQuota.php