This filter helps to modify the image size of the gallery.

Source

File : /inc/helpers.php

Example :

function wp_travel_thumbnail_size($sizes){
 $sizes['width'] = 250;
 $sizes['height'] = 250;
 return $sizes;
}
add_filter('wp_travel_image_size','wp_travel_thumbnail_size');
function wp_travel_gallery_image_size( $image_size ) {
    return 'wp_travel_thumbnail';
}
add_filter('wp_travel_gallery_image','wp_travel_gallery_image_size');