Past research has investigated the impact of mutual knowledge on communication by focusing mainly on verbal communication. This study uses a wider focus, which includes speech and gesture. Speakers completed a referential communication task with recipients who did or did not share with them knowledge about the size of certain entities. The results showed that when such common ground exists between interlocutors, speakers' use of gesture and speech is affected. The main finding was that when speakers talked to recipients for whom the size information was new information, they represented this information predominantly in gesture only or in gesture and speech. However, when speakers talked to recipients with whom they shared knowledge about the entities' size, speakers encoded this information mainly verbally but not gesturally. The results are interpreted with respect to past research into common ground and language use, the pragmatics of gesture, and theories of gesture production.