Wordpress - Are there ways to make the Gutenberg editor wider? And the HTML-block higher?

You have to enqueue first the style editor css

function legit_block_editor_styles() {
wp_enqueue_style( 'legit-editor-styles', get_theme_file_uri( '/style-editor.css' ), false, '2.3', 'all' );} 
add_action( 'enqueue_block_editor_assets', 'legit_block_editor_styles' );

Then you have to create that style-editor.css file inside your theme and then add this inside that file:

.wp-block {
max-width:  100%;}