Adding a new image size to WordPress Functions file

// Add additional image sizes
    add_image_size( 'homepage-thumb', 535, 350, true );

// Show new image size in admin are of media section
add_filter( 'image_size_names_choose', 'yourTheme_custom_image_size' );

function yourTheme_custom_image_size( $sizes ) {
    return array_merge( $sizes, array(
        'homepage-thumb' => __( 'Homepage Thumbnail' ),
    ) );
}