Create thumbnail size only for a few images?

Yes, you can choose to regenerate the thumbnails for specific images. You have 2 options.

1- Create a simple plugin

Generating thumbnails is done via the wp_generate_attachment_metadata() function, which you can use to generate thumbnails manually. To do so, make a simple plugin that loops through a couple of attachment IDs and generates thumbnail for them. Here’s what you need:

<?php
/**
 * Plugin Name: Generate Thumbnails
 * Plugin URI: http://example.com/
 * Description: A plugin to generate some thumbnails.
 * Version: 1.0
 * Author: Desi
 * Author URI: http://example.com
 * Text Domain: desi
 *
 */

// Trigger our function once the plugin has been activated
register_activation_hook( __FILE__, 'wpse335041_regenerate_thumbnails' );

function wpse335041_regenerate_thumbnails(){
    // This is an array of attachment ids
    $attachment_ids = [ 14, 490, 77, 129 ];

    // Loop through the ids and generate thumbnail for each
    foreach ( $attachment_ids as $id ) {

        // Check if the attachment is an image
        if( wp_attachment_is_image( $id ) ){
            // Get the attachment path
            $attachment = get_attached_file( $id );

            if( $attachment ) {
                // Generate thumbnails and metadata
                wp_generate_attachment_metadata( $id, $attachment );
            }
        }

    }
}

All you have to do now is to activate the plugin. If there are lots of images that need regeneration, it might take a while.

2- Use a plugin

There’s a well-known plugin in the plugin repository that can be used to regenerate the thumbnails for a single or multiple image. You can download the plugin via the plugin repository here. Notice that I’m not the plugin’s author.