The paper examines the impact of class size on postgraduate grades using administrative data from one of the largest Schools of a Russell Group University in the UK. As well as estimating Fixed Effects models on the population of postgraduate candidates in the School, we exploit a policy change aimed at reducing class size to implement a regression discontinuity design (RDD). We find that class size does impact grades adversely overall; and the policy aimed at reducing class size impacts grades favourably. Our findings are robust to alternative specifications as well as being supported by the validity tests we conducted.