I wrote a bash script to get mailbox size of all mailboxes and export it to csv. Hopefully someone finds this useful.

I would post it to the wiki put when I created by wiki account I didn't see a create page link.

Code:
#!/bin/bash

#This script exports email address & mailbox size in MB to csv for all accounts
#It was tested against a single domain setup
#For 125 accounts and 300GB total volume size it took about 5 minutes
#By James Barnett james.barnett@noc.com

cd /home/zimbra;

echo "This will take a few minutes ..."

zmprov gaa > temp.txt; #export a list all accounts to a temp file

rm -f size.csv #remove the any pervious CSV file

echo -n "Email Address, " >> size.csv 

echo "Size of Mailbox in MB" >> size.csv #echo column headers to the CSV file

for i in $(cat temp.txt); #loop through the accounts
do
     echo -n "$i, "  >> size.csv; #echo the email address

     bytes=$(zmmailbox -z -m $i getmailboxsize -v) #get mailbox size in bytes and set to variable

     echo "scale=2; $bytes/1048576" | bc>> size.csv #return mailbox size in MB
done

rm -f temp.txt #remove temp file

echo "Finished! Check /home/zimbra/size.csv for results"

echo -e "\a" #Ring the system bell to let the user know the report is ready