Register custom image sizes in WordPress

wp-custom-image-sizes.php

<?php

// Add featured image sizes
add_image_size( 'custom-small', 500, 300, true ); // name, width, height, crop
add_image_size( 'custom-large', 1840, 793, true );

// Register the image sizes for the Add Media modal
add_filter( 'image_size_names_choose', function( $sizes ) {
    return array_merge( $sizes, array(
        'custom-small' => __( 'Custom Small' ),
        'custom-large' => __( 'Custom Large' ),
    ) );
});