Children's books come in all sizes. These are some of the smallest I have found that fit my "children's non-fiction" about space collection. These were part of the "Orbit Pocket Library" and are only 10 cm by 10 cm.
Rockets and Spacecraft, Book One. and Two London : Collins. (28 p.) 10 cm. Board, DJ.
With no obvious copyright date, from spacecraft depicted it is around 1966. Has text alternating with illustrations. They have many illustrations of rockets, both military and various civilian programs.
Especially interesting are the British rocket designs depicted. It is almost impossible to find children's books about the British rockets.
as well as American concepts that never quite got beyond the planning stages.