This study was carried out mainly to investigate the effects of class size and self-esteem on student achievement in—and attitude toward—English essay writing. The study was influenced by earlier research reports indicating that the large class sizes prevalent in Nigerian schools may be detrimental to student academic and emotional well-being. The study employed an ex post facto research design, as no attempt was made to alter the pre-existing conditions in the schools. Three hundred and thirty-five (335) Senior Secondary School 2 students from nine purposively selected schools participated in the study. An achievement test in essay writing (r = 0.7); attitude toward essay writing questionnaire (r = 0.72); and self-esteem questionnaire (r = 0.84) were the instruments used for data collection. The data collected were analyzed using analysis of covariance. All hypotheses were tested at a 0.05 level of significance. Results showed that there was a significant effect of class size on student achievement in essay writing, whereas class size had no significant effect on attitude toward essay writing. Results also showed that there was no significant effect of self-esteem on student achievement in—and attitude toward—essay writing. Based on these findings, it was recommended that the government should ensure that classrooms are not overcrowded.