If you want to do a cleanup of your archive (or standard mailbox too) you can do the following:
In outlook, go to File -> Tools, select Mailbox Cleanup
If you want to find large items or old items, select items older than x.. or larger than x and click Find
Make sure we're working in your archive folder by choosing Browse...
Uncheck your main account
Click ok
Click find now
Click a message in the lower pane of the search window, then press ctrl+a to select everything
Right click a message in the list and choose delete
This should delete everything older than x days from your archive