Could have tall and thin books or short and fat books. Each would change the shape of the illustrations and what you could show in them. For example if the story was about a giant, a tall and thin book would easily show the whole giant, while a short and fat (landscape, basically) book might only show the giant's feet.
Children's books are often over-sized. However, Beatrix Potter's books were often tiny - for tiny hands.
An earlier post inspired me to make a series of matchbox-sized books.
If I was to make a very tiny book, I could make tiny illustrations that the viewer would need a magnifying glass to see properly. Would be fun!
Also consider shaped books - different constraints to the regular 4 sided shape.
