find /path/to/mail/store/ -mindepth 4 -maxdepth 4 -type d \
|fgrep spam |xargs -i du -s {} \
|awk '{print $1$2}' |awk -F/ '{print $1"\t"$9}'
Where /path/to/mail/store is the mail store path, and spam is the name of the users’ spam boxes. Useful also for INBOX, drafts, and other standard boxes.
The command returns the number of kilobytes a box takes up, followed by the username of the box’s owner.