A gallery field can return the URL, ID or an array. To show a specific size image you need to return something other than URL. If you return the id then you would use https://developer.wordpress.org/reference/functions/wp_get_attachment_image_src/ to get the image. If you return an array see the documentation of the image field to see what is returned for each image in the array https://www.advancedcustomfields.com/resources/image/.
