You may use the Mailbox Cleanup tool to manage the size of your Mailbox. Go to the File tab and click Mailbox Cleanup button and  select Mailbox Cleanup:

This tool will allow you to:

• View your Mailbox Size
• Find items older than a certain amount of days you choose
• Find item larger than a certain amount of kilobytes you choose
• Auto Archive your mail by clicking on the AutoArchive button it will move old items to the archive file on your computer. You can find the archived items under Archive Folders in the folder list displayed with your Mail Folders.
• Empty your deleted items folder permanently
• Delete all alternate versions of items in your mailbox