ISPConfig has plugins only in the daemon part that runs on the servers and not in the interface. But you can add new functions to the interface too of course, they are just not plugins.
Implementing this is not that easy as ispconfig is a multiserver controlpanel, so you can not assume that the interface is on the same server then the websites. This means that the best place to add the code that does the actual monitoring is in the server/mods-available/monitor_core_module.inc.php file. This code has to store the size of the websites in the mysql (master) database like the other monitoring data. Then you will have to add some code in the interface/web/sites/web_domain_edit.php file to display the quota or you add it to a separate file like it is done for the mail traffic.