The broad objective of this study is to examine audit firm size and audit quality in Nigeria. The longitudinal research design was adopted for this study. The data used in the study were obtained from 540 firm-year observations, comprising of sixty (60) Nigeria listed companies listed from the period of 2007- 2015. Model on the input-based measure of audit quality was adopted in the study. Descriptive statistics and regression analysis were carried out with the aid of E-views 8.0. From the result, audit firm size was found to have a positive relationship with audit quality. Implying that the Big 4 audit firms tend to have a more far-reaching client base and cannot help but render quality audit to ensure clients retention and public confidence. The Big 4 audit firms have the resource base to invest in new and qualitative technology which will prompt the quality audit. We recommend that the non-Big 4 audit firms should pursue expansion to increase their resource base and the capability to deliver a good quality audit.