If you are getting 0 MB size for MySQL database in your cPanel account. Then you can resolve it through below given steps.
1). Login to SSH as root.
2). Open cpanel.config file through vi editor
3). Now, look for
4). You have to change the value to 1 from 0
5). If this parameter is not present in the file then you have to add it.
6). Save the file and execute the following command:
It may take some time to get it fixed depending on number of users with databases. Once it is done then you will be able to see database disk usage show up accurately in cPanel.