The variation in legal system quality across states in Mexico is used to examine the relationship between judicial quality and firm size over the course of the 2000s, when systemic changes were taking place. Using economic census microdata and survey‐based measures of legal institutions, a robust effect of judicial quality is observed on the firm size distribution and efficiency, instrumenting for underlying historical determinants of institutions. Indicative evidence is found that the effect is strongest in more capital‐intensive industries. Market size and distance‐to‐market are also found to matter for firm size outcomes, consistent with the new trade literature.