This paper presents phonetic and phonological evidence for size restrictions on prosodic structures in Taiwan Mandarin, based on new experimental data. Several studies have argued that there are binarity restrictions at different prosodic levels (e.g., Prince 1980; Ito and Mester 1992; Selkirk 2000). This paper provides new evidence from Taiwan Mandarin to support this claim. This paper argues that syntactically flat structures like /wuL wuL wuL wuL wuL/ '55555' must be decomposed into Major and Minor Phrases. Essentially, Major Phrases in Taiwan Mandarin are obligatorily binary, consisting of two Minor Phrases. The binarity restriction of Major Phrases accounts for the patterning of several phenomena in Taiwan Mandarin: glottal stops/glottalization, pauses, and tone sandhi. Finally, this paper argues that the binarity restrictions of Major and Minor Phrases are due to constraints which delimit the lower and upper bound of relevant prosodic domains.