It might be your cache issue. Kindly clear your site and browser cache and then check back the product page.
You can use the equal resolutions for all of your images. So they all will be in equal sizes. There is no such option to display all the images of the same size.But image should be big enough to have a nice view. Just for information we provide images sizes applied in our demo:
Product main image: width: 800px; height: 1200px;Blog post: width: 1024px; height: 720px;Page title background: 1200px; height: 400px;
This is not the theme issue. I think the products not imported properly. If it was the theme issue then the button should work on the other products as well so it is your import issue. Please ask your developer to check or import the products properly on your site.