A consistent body of research shows that large classes have been perceived by teachers as an obstacle to deliver quality teaching. This large-scale study sought to investigate further those differential effects by asking 1,119 teachers from 321 K-12 schools in New South Wales (Australia) their perceptions of ideal class size for a variety of student cohorts. In general, there was unanimous support for smaller class sizes particularly for the earlier years of schooling. Teachers also recommend small classes for diverse cohorts such as students from low socio-economic, low and high academic performance, non-English speaking, gender, Indigenous, rural background and a range of learning exceptionalities. This study suggests that a more multidimensional view of "optimal" class sizes as a factor to improve student learning, one that requires to embed student diversity into research methodologies. It also recommends that class size reduction programs should be implemented under the provision of new instructional strategies, pedagogically productive to smaller settings, are to be enacted.