How do I find and select the small version of the softcover book in iPhoto?

Click on Change Version and then on the Softcover button at the top. Finally look at the lower left hand cornder of the window for this:

I would made a duplicate of of the book before changing size or themes. Click on the book in the left hand pane and type Command + D (Duplicate). Use the copy to change size and be sure to check all text boxes carefully as some may not be big enough to contain all of the text from the larger size and give a text overflow warning sign:

OT