zmmailbox -z will initiate an interactive session. Then use the command sm <account@domain> to select the mailbox you want to examine. You'll immediately get a summary showing the size of the mailbox, the number of messages, and the size. When you're done, you can exit.
You'll have to experiment & see exactly what constitutes a "message". I.e., it might just be emails, but it might include appointments, documents, etc. |