Add Custom Images Sizes to Gutenberg Image Block

functions.php

<?php

add_filter( 'image_size_names_choose','wpmudev_custom_image_sizes' );

function wpmudev_custom_image_sizes( $sizes ) {
return array_merge( $sizes, array(

//Add your custom sizes here
'custom-image-square' => __( 'Square' ),
'blog-width' => __( 'Blog Content Full Width' ),
) );
}