Cellulosic ethanol made from cellulosic biomass is a promising alternative to petroleum-based transportation fuels. Enzymatic hydrolysis is a crucial step in cellulosic ethanol production. In order to better understand the mechanisms of enzymatic hydrolysis, relationships between cellulosic biomass particle size and enzymatic hydrolysis sugar yield have been studied extensively. However, the literature contains inconsistent reports. This paper presents an analysis of the inconsistent reports on the relationships in the literature. It discusses the differences in the reported experiments from five perspectives (biomass category, particle size definition, sugar yield definition, biomass treatment procedure, and particle size level). It also proposes future research activities that can provide further understanding of the relationships. •We summarize reported relationships between biomass particle size and sugar yield.•We summarize experimental conditions and results on this topic.•We analyze the inconsistent relationships from five perspectives.•Several hypotheses are formulated based on what is reviewed.•Future research is proposed to test these hypotheses.