Easyslider Fullwidth custom ratio

Hi !
I’m struggling to get a good solution for creating a slider that is ok with both horizontal and vertical images. With the regular easy slider, vertical images occupy to much space in the page. So I was figuring to use a fullwidth easyslider with a constrained ratio and a lightbox, so that people who want to see the original vertical image can.

Problem is the 1500×630 ratio is to stretched. 3:2 photo ratio who imply a 1500×1000 px. I search the forum and found this code :

// add new size
add_image_size( ‘my-size’, 1500, 700, true );

// make the new size available in the media library (and image element etc.)
add_filter( ‘image_size_names_choose’, ‘my_custom_sizes’ );

function my_custom_sizes( $sizes ) {
return array_merge( $sizes, array(
‘my-size’ => __( ‘My Size’ ),
) );
}

I tryied it but it actually makes the same thing as if I was using the “no scalling” option. My vertical images are way too big !

Any solution for customizing this ? Thanks !