use a Thumbnail size in post

You can use regex if you want to display it on all existing posts.
WordPress add a wp-image-ATTACHMENT_ID class on images. We can use it to find the attachment_id then grab the new url. For example here, with medium size.

wp_get_attachment_image_src : https://developer.wordpress.org/reference/functions/wp_get_attachment_image_src/

We apply this filter to the content to update the display of your posts. We use regex to find attachment_id and image src attribute.

function wpse_289055_new_image_size($content) {
    preg_match('/wp-image-[0-9]*/', $content, $matches);
    if($matches !== NULL)
    {
        $thumb = wp_get_attachment_image_src(intval(str_replace('wp-image-', '', $matches[0])), 'medium');
        if($thumb)
        {
            preg_match('/ src="https://wordpress.stackexchange.com/questions/289055/([^"]*)"https://wordpress.stackexchange.com/", $content, $matches);

            if(isset($matches[1]))
            {
                $content = str_replace($matches[1], $thumb[0], $content);
            }
        }
    }
    return $content;
}
add_filter('the_content', 'wpse_289055_new_image_size');

In my opinion if you don’t use a cache system, this code use some ressources. Adapt this one to update all your existing post directly.