Travelo theme has not such feature. You should use same size images for slider in Travelo theme.
Or please try to following.
1. Please add following code in bottom of /functions.php file in Travelo theme directory
add_image_size( 'slider-gallery', 1200, 600, true );
You can change 1200 and 600 based on your image size yourself. It is image width and height.
2. Please find following code in /single-accommodation.php file in Travelo theme directory
wp_get_attachment_image( $gallery_img, 'full' )
And replace with following.
wp_get_attachment_image( $gallery_img, 'slider-gallery' )
3. Find following code in /inc/functions/functions.php file in Travelo theme directory
$src = wp_get_attachment_image_src( $gallery_img, 'full' );
and replace with following
$src = wp_get_attachment_image_src( $gallery_img, 'slider-gallery' );
4. Install "Regenerate Thumbnails" plugin and regenerate thumbnails with this plugin.
And then visit your website again.