Dont know an ideal way but this works...
Click on the folder so that the mails are then shown in the main view. Click File-Export, and select comma delimited. Give the file a name (e.g exported.csv) and choose to export all documents. You can then open the csv file in Excel (or other spreadsheet app) and do a sum on the size column.
Like I said, dirty and drawn out, but it works if its a one off task.