If you need a rather quick way to export all the mailbox sizes on a particular server then the below link is for you.
Download the script, run it by opening the command prompt and type cscript MSX_GetMailboxSize.vbs thenameoftheserverhostingthemailboxes>filenameyouwanttochoose.csv
Remember to add the export location to the above command if you want to save the export to a particular path ie: cscript MSX_GetMailboxSize.vbs servername>e:reportsserver1.csv
Open the csv export with Excel after you ran the script. You can then manipulate the contents to the format you need as all mailbox data will be there.
Fields exported to csv using this script are:
I used this to work out an average mailbox usage per server.
I found the script here. There’s also more information on this script from the source site.
Hope this has been helpful.