Add-Custom-Sizes-theme-functions.php

<!-- Add this to your theme functions.php file -->
/*
		 * Enable support for Post Thumbnails on posts and pages.
		 *
		 * @link https://developer.wordpress.org/themes/functionality/featured-images-post-thumbnails/
		 */
		add_theme_support( 'post-thumbnails' );

		// This theme uses a custom image size for featured images, displayed on "standard" posts.
		if ( function_exists( 'add_theme_support' ) ) {
			add_theme_support( 'post-thumbnails' );
			set_post_thumbnail_size( 150, 150, true ); // default Post Thumbnail dimensions (cropped)

			// additional image sizes
			// delete the next line if you do not need additional image sizes
			add_image_size( 'big-img', 870, 500, false ); // Size for default post full width
			add_image_size( 'big-thumb', 539, 380, true ); // Size for 2-up thumbnails
			add_image_size( 'arch-thumb', 418, 240, true ); // Size for 2-up thumbnails
		}

		$image_size = 'big-thumb';
		$image_url = wp_get_attachment_image_src( get_post_thumbnail_id( get_the_ID() ), $image_size );
		$image_url = $image_url[0];

		// Add our custom sizes to the Dashboard choices
		add_filter( 'image_size_names_choose', 'my_custom_sizes' );
		function my_custom_sizes( $sizes ) {
			return array_merge( $sizes, array(
				'big-img' => __( 'Big Image 870x500' ),
				'big-thumb' => __( 'Big Thumb 539x380' ),
				'arch-thumb' => __( 'Archive Thumb 418x240' ),
			) );
		}