When placing photos in a book template how can I reduce the size to fit

You probably need to use a different theme. The option "fit to frame" is not available for all themes, see:

Arrange photos on a book page in Photos on Mac - Apple Support

Fit an entire photo within a frame: Control-click the photo and choose Fit Photo to Frame. (This command isn’t available for all themes.)

Try the "Picture Book" theme , for example. In "Simple Shots" you cannot fit the photos to the frame.

Many themes have fit to frame (place the photo and right click - control click - on it and select fit to frame

Some frames do not so if the theme you are using does not have fit to frame then you will have to either change themes or duplicate the photos in Photos (command D - which does not create a second photo but just creates a second non-destrective edit list so it does not increase disk usage) and crop to the correct ratio

LN

Also in that same menu you can edit the photo in Photos editing suite. That let's you do most adjustments while still in the book module.

Right! And while you are in edit mode, you can open the Info panel of the photo. This let's you see the title, the place, the date of the photo, and you can copy and paste the title to the text field on the book page. It is the only way to access the metadata of a photo, while you are working on a book and want to add captions to the book pages.

you are welcome - enjoy your book and have a great week

LN