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<?php

function woocommerce_get_image_size_gallery_thumbnail_override( $size ) {
	return array(
		'width'  => 300,
		'height' => 400,
		'crop'   => 0,
	);
}
add_filter( 'woocommerce_get_image_size_gallery_thumbnail', 'woocommerce_get_image_size_gallery_thumbnail_override' );