I think thunderbird is able to achieve this but can I check the size of my IMAP folders in Outlook 2010 or via a small program?
In Outlook 2007 you can right-click the IMAP folder and choose Properties. In the General tab of the Properties dialog is a button labeled "folder size" which, if you click it, will tell you the current folder's size, with and without sub-folders included.
In Outlook 2010, right-click the IMAP folder and choose Data File Properties. In the General tab, click the Folder Size button.