How to write an identified code

Hi, I've identified a code in web developer that I want to change but I don't know how to write it & whether to put in CSS or page settings. Here's the link https://sandra-murphy-kkk7.squarespace.com/autumnwinter-2013/& see examples attached, I want to adjust a style height from 96.3999999% to 100%. Originally I was trying to get the book centered on the page at that size (width="700" height="500") but I can live with it how it is if I can get rid of the advertising links below. Thanks, Sandra.