I guess I wasn't explicit enough. I assumed people already knew how the quota's are stored. The "user quota" for vpopmail is stored in the pw_shell attribute of the vqpasswd structure. Where this information is stored (db, cdb, file) doesn't matter. You use an API to get at it. In courier, a different place is used. The "domain quota" is now currently stored in the qmailadmin limits information, which is now retrieved using the vget_limits() function. Again, where it is actually stored doesn't matter.
The important thing to remember is that Sam's approach now is to set the quota with maildirmake -q. The quota is stored ONLY in the maildirsize file itself. It is the responsibility of any Maildir++ aware applications to honor the quota established in that file.
I only used pw_shell for backwards compatibility with the old vpopmail quotas.