Rambam chametz umatza 6, haloacha alef: Umisheachal kezayit yatza.
What you do after you are yotze? Why delay the next mitzva with stam eating?
and to the original subject, nowhere does the Rambam specify the size of the kezayit he cites, because it was obviously the size of the matzui olive. not needing further elucidation.