Woocommerce gallery images size to full size

Hi, I have storefront&woocommerce with the plugin for ‘Additional Variation Images’ which works to allow me to select a product variable image (product detail page) for each product variable without swapping the main image (I wanted the gallery image to change, not the main). This works fine, however the gallery image is about 25% the size of the main product image, as there is room for up to 4 gallery images per product (4 columns). I only need 1 image displayed at full size (same as the product image above), so can anyone help with some custom CSS to control this? (I am using a child theme)

Site with example product on: https://equitility.com/dev/?product=balloons-b60-sk

The style appears to be: .attachment-shop_thumbnail