This paper explores mathematics teachers’ perceptions about class size and the impact class size has on teaching and learning in secondary mathematics classrooms. It seeks to understand teachers’ views about optimal class sizes and their thoughts about the education variables that influence these views. The paper draws on questionnaire responses from 83 secondary mathematics teachers as well as interviews with 12 of these teachers. Although this current study focuses on mathematics teachers, it is nested in a much larger study across all subject areas. This present study found that class size directly impacts the attention teachers pay to individual students, the amount of investigative work undertaken and classroom management practices particularly for low achievers. However, other variables such as student ability, grade level and teacher quality make it almost impossible to determine what could be regarded as an ‘optimal’ class size. Views were also expressed about the advantages and disadvantages of smaller class sizes in terms of classroom management, class cognitive and social interaction, and teacher professional development. Implications of class size for specific student characteristics such as gender, Aboriginality, gifted and talented, socio-economic and language background issues are also intertwined in the discussion.