Is there an API that allows a developer to get the disk usage info of a particular virtual host/domain?
I could not find a dedicated API command for that, but I suppose you write a shell script utilizing the Linux du command on the required directories, which would be public_html for web page data, homes for user mailboxes and domains for sub-servers.
Also, you can find out which servers and users exist and what directories they have using
virtualmin list-domains --simple-multiline
virtualmin list-users --all-domains --multiline
Thanks @Locutus for the info. I guess I have to write my own script for this purpose.
Yeah, with the powerful stuff that bash can do, it shouldn’t be too hard. 
But maybe the developers here can take this as a request for improvement, and will add such a script in Virtualmin some time.