Does ISPC3 offer a way to limit the size of MySQL databases or does every database have unlimited ammount of space it can use?
There are no mysql quota limits as mysql does not offer to limit the site of a database and linux filesystem quote does not work for this purpose as well as all databases use the same linux user "mysql", so you would be able to limit only the size of all mysql databases but not a specific mysql database.
Maybe you could use this - although it is not integrated into ISPConfig it might fit your needs. Maybe. http://lrem.net/software/mysql-quota-daemon.xhtml
Wouldnt it be possible to add some kind of soft-limit? It might not be possible to limit the size of dbs but the size can be calculated. This value could be compared to the "limit" for the user. If the size is greater than the limit new dbs could be denied. A notification could be added as soon as notifications are implemented into ISPConfig.