Folder Size (Outlook 2010)
Here is a great youtube video to help you find which folders are using up the most space in your email:
NOTE: If you are archiving, your "Data File Properties" will show 2 tabs ("Local Data" and "Server Data"). In this case you should only be concerned with the size of folders under the "Server Data" tab - this is what could keep you from sending/receiving emails if your box is too full.
NOTICE: Unfortunately, this does not apply to Outlook Web Access