This is done with alternative templates
You can make them through the visual theme editor by using the top-middle dropdown menu.
Then if your doing this with only CSS styles and your theme is an online-store2.0 theme use a custom-html or custom-liquid section to insert specific CSS in a style tag.
Otherwise you need to find this newly created template in the theme code editor and modify what size images are rendered.
Merchants that want this customization can contact me at firstname.lastname@example.org with topic url, store url, theme name and relevant details.