To get to this window, click on Tools \ Mailbox Cleanup.
In this window you have several options to choose from. The easiest option is to select AutoArchive, which will move older items into the Archive Folder. You could also choose to empty the deleted items.
One of the more useful features however, is to quickly find all large emails using the “Find items larger than” button. Deleting all the largest emails would very quickly bring your inbox size down to a reasonable level.
