Where are the mail mate mailboxes kept on my Mac??

You can get the total size doing the following in the Terminal application:

du -h -d0 ~/Library/Application\ Support/MailMate

You can also navigate to the folder using Finder, but note that ~/Library is hidden on Mac OS X Lion by default.

In my case, the space used by MailMate is about the size of all my messages plus 10% (index files).

Note that it is always safe to delete the following folder:

~/Library/Application\ Support/MailMate/Attachments

It is a cache used to save attachments before viewing them. The original messages still contain these attachments. It is not safe to move/delete most of the other files.