Set the 'Limit Size' to leave empty space on the drive, AND, increase the 'Limit clip age' to the number of days you want to keep.
BI will limit according to whichever comes first.
Also, you really don't have to move to Stored. Just delete them (NOT Recycle). Less overhead.
Caution: According to the BI Help, you should not fill the hard drive...
The size specified with "limit folder size" should fit comfortably on your hard drive. If you are short on space for it, you will see red warning text at the bottom of the window as well as status/messages log entries with double exclamation points !!. You should not allow your drive to fill up as this may cause video and/or database corruption.