The girdle book is one that is made with a cover that is long on one end and can be tucked into a girdle or belt. You might notice that they are upside down, so that you can read them when you pull them upward from below and don't remove them from your waist. 

there are instructions on the web about creating girdle books using covers for your modern book or actually pasting the cover onto the book.

This is an extant girdle book.

I'm not sure how small it is, but it seems more likely that it is small and thick, not large and thick. It is supposed to be carried, so how large would you want it to be?

Here is one that is smaller.

I like the locks.

Here is the same book, open.

very rough at the edges.

This is a different book.

Again, very rough leather.

Get a good idea of the size from this. This is an extant girdle book. Very small.

Smaller than a paperback.

Here is a picture of a person holding a girdle book. (from a manuscript)

You can see that it is about the size of a paperback.

St. Anthony

here is a blue one!

The pattern below and the link are both the creation of Jaysen Ollerenshaw.

Here is a great site for making your own. With a pattern.