Source code for https://victorfont.com/add-custom-image-sizes-to-media-library/

add_image_size_syntax.php

<?php

add_image_size( string $name, int $width, int $height, bool|array $crop = false );

add_new_image_size.php

<?php

//* Add new image sizes
add_image_size( 'front-page-icon', 90, 90, false );

function post_image_sizes($sizes){
	$custom_sizes = array(
		'front-page-icon' => 'Front Page'
	);
	return array_merge( $sizes, $custom_sizes );
}
add_filter('image_size_names_choose', 'post_image_sizes');

add_theme_support.php

<?php

if ( function_exists( 'add_theme_support' ) ) { 
    add_theme_support( 'post-thumbnails' );
}