This study empirically assessed the relationship between school size and academic achievement of elementary school students in Ontario, Canada. Utilizing data from the Ontario provincial standardized test, the Educational Quality and Accountability Office (EQAO), the results of 541 schools from ten school boards, were studied. A One-way Analysis of Variance (ANOVA) indicated that overall, there was no statistically significant correlation between school size and student achievement. However, there were significant correlations with respect to levels of performance in both Grades three and six in some curricular areas. Also, further analysis at each independent achievement level revealed that the mean percentage of students achieving at stipulated provincial standards in Grade three writing and in Grade six reading, writing and mathematics were highest in large-sized schools (schools with more than 420 students). Results further indicated that the mean percentage of students performing above provincial standards in Grade six reading and writing was also highest in large schools. Students in medium-sized schools (between 246 and 420 students) also had the highest mean percentage of students performing above provincial standards in Grade three writing and in Grade six mathematics. The limitations and implications of the results are discussed, and relevant suggestions made.