Does anyone happen to know what is the physical size impact on an Active Directory structure receiving classes/attributes from an SCCM schema extension?

Obviously it depends on size and scope of the company, but let's say the company has 50,000 clients. How much space will active directory need to accommodate all of the clients/sites/DPs/SUPs/Packages/etc?

Almost nothing. The only thing that get added to AD are boundaries nothing else. So the more boundaries you have the more entries

No, client records are written to the ConfigMgr database (not the Active Directory database). This is the container you were referring to. I have plenty of clients in this environment but no sign of them here.

No, client records are written to the ConfigMgr database (not the Active Directory database). This is the container you were referring to. I have plenty of clients in this environment but no sign of them here.