A product image displayed in the Details Panel uses the woocommerce_thumbnail size. This size is used in the product grids in places such as the shop page. Read more about WooCommerce Image Sizes.
You can customize this size in the customizer. Learn more about this in the WooCommerce Documentation. Remember that these changes will affect both the Details Panel and the product grids in places such as the shop page.
How to change image size in the Details Panel only?
You can do this by creating a new image size.
add_action( 'init', function () {
add_image_size( 'dgwt-wcas-product-custom', 300, '', false );
} );
add_filter( 'dgwt/wcas/suggestion_details/product/thumb_size', function ( $size ) {
return 'dgwt-wcas-product-custom';
} );
ⓘLearn how to add this snippet to your WordPress.
After that you can regenerate all thumbnails. We recommend the Regenerate Thumbnails plugin for this purpose.