No Dick it's the size in bytes, if after a compact and repair it say 1mb then when it gets to say 2mb do another and see what the size is now.
BTW I am not suggesting doubling the size is ok it was just a simple example , you would not wait for it to go from 10 gig to 20 .
I am sure that you would soon get a feel for the frequency required depending on the activity on the DB.
But then doing it everytime may be the simpler option.
Brian