Many theories of memory propose some type of short-term store limited in capacity to a small number of information chunks. However, although short-term verbal memory is generally considered to be a crucial component of language processing, the relevant information chunk level that may define capacity limits in ecologically-valid spoken language has never been investigated. The Intonation Unit (IU), an intermediate-level prosodic phrase, has been theorized to be a fundamental unit of spoken language, the focus of a speaker's mental processing. This suggests that IUs might play a role as the relevant unit representing "chunks" of spoken language. We report the results of an experiment investigating the role of IUs in short-term memory in a serial recall task. We found a significant non-linear effect of stimulus size in IUs, but not clauses. We conclude that Intonation Units are the primary linguistic unit used for chunking spoken language input in memory.