Use 12pt

main.tex

@@ -1,5 +1,5 @@
 % Template
-\documentclass[twoside, a4paper, 11pt]{book}
+\documentclass[twoside, a4paper, 12pt]{book}
 \usepackage[inner=35mm, outer=20mm, top=30mm, bottom=25mm]{geometry}
 
 \usepackage{setspace}