What is the usual InDesign margin used for printing A4 books, and how do I recalculate it for A7?

It is probably a good idea to examine popular books in your specific field, with a ruler. A university textbook, a children's picture book, a book about wine, and an atlas, are likely to have different customer expectations. Miniature books may be decorative or densely packed.

I suggest you set up a couple of specimen pages and try different margins, fonts, sizes and leading to establish which is the best (most suitable) combination before committing yourself – use Paragraph Styles to make changes to fonts etc easy.

There isn't really any "usual" margin, at least not a single generic one. It depends on the content, usability, readability, esthetics, etc.

The minimum margin for normal book pages is usually 1/2 inch, 3 picas. Add to the inside/binding margin; 4pc is probably about right. (I usually set up 6x9 pages using a top header with 4pc top, 3pc outer and bottom, and 6pc inside for perfect binding.)

On these very small pages, you could go to 2 and 3.5 if there will be no trim variations. Any smaller is a judgment call.