functions.php

// 1. Register the new image size(s)
function custom_bb_image_sizes() {
    // Syntax: add_image_size( 'unique-slug', width, height, crop_boolean );
    add_image_size( 'grid-landscape', 600, 400, true );        
}
add_action( 'after_setup_theme', 'custom_bb_image_sizes' );

// 2. Make the sizes selectable in Beaver Builder & WordPress Media Library
function custom_bb_image_size_names( $sizes ) {
    return array_merge( $sizes, array(
        'grid-landscape'     => __( 'Grid Landscape' ),        
    ) );
}
add_filter( 'image_size_names_choose', 'custom_bb_image_size_names' );