Graphics sizing and positioning for book layout

Your question is sort of hard to answer because book layout, page layout, is subject to a spectrum of rules from the very rigid (mostly deriving from printing and readability needs) to traditional (proportions of margins, placement of things like headers, footers and page numbers) to what might be called just esthetic choices of the designer. There aren't really any fixed rules for that latter category.

Your best bet is to find a few books of similar content, preferably from major authors and publishers, and study how they place images, captions, text references and the like, and use that as guidelines. In general, you want a pleasing balance of text and illustrations, with no crowding, and neither too much open or white space nor too little. If the page looks right to your eye... it's probably good enough. (And, of course, have others with some experience and "eye" for book content look your pages over, especially those who might be readers of the material.)