What is the ideal trim size for baby books? An average of six-by-six inches is what I see most often. It’s a good size for perusing, carrying, and even stacking by board books’ very young audience. I wonder if this convention harks back to Beatrix Potter, whose own picture books measured just under six inches high and who famously thought that children’s books should be small for little hands. But not all board books use this standard trim size. Sometimes publishers produce them at a height of ten inches or more. Some opt for a squatter stature of five inches or less. And sometimes board books are printed in editions both big and small.