Añadir Tamaño de las imágenes personalizado

Añadir a la biblioteca de medias de WP tamaño de imagen personalizado:

function eb_after_setup_theme () {
    add_image_size( 'content-slide', 847, 350, true );
}
add_action( 'after_setup_theme', 'eb_after_setup_theme' );

function eb_image_size_names_choose( $sizes ) {
    $custom_sizes = array(
        'content-slide' => 'Content Slide'
    );
    return array_merge( $sizes, $custom_sizes );
}
add_filter( 'image_size_names_choose', 'eb_image_size_names_choose' );

Consultar:

https://developer.wordpress.org/reference/functions/add_image_size/