Despite consensus in published studies that larger-breasted patients who undergo radiation therapy tend to suffer from more severe acute skin reactions and a more adverse cosmetic outcome, there appears to be no consensus on the definition of a ‘large breast’. This paper describes an analysis of breast size that was undertaken on 50 patients and compares this data with other published studies. The desired outcome for the study was to formulate a definition of ‘large breast size’ that would be appropriate for Australian patients and that could be determined simply and quickly in busy radiation oncology clinics. Analysis suggests that cup and brassiere size may be used to separate breast size into two categories. On the basis of published data and results from this study, it is recommended that patients with a cup size ≥D or a bra size ≥18 could be categorized as having large breasts, with all other patients considered average in size.