Here is what I did:
functions.php
/* Editor styles EXTENDED */
add_action('admin_enqueue_scripts', 'load_admin_style');
function load_admin_style() {
wp_enqueue_style('admin_css', get_stylesheet_directory_uri() .'/gutenberg-styles-extra-wide.css' );
}
gutenberg-styles-extra-wide.css
/*--------------------------------------------------------------
# 1.0 - Editor
--------------------------------------------------------------*/
body.block-editor-page .edit-post-visual-editor .editor-post-title__block,
body.block-editor-page .edit-post-visual-editor .editor-default-block-appender,
body.block-editor-page .edit-post-visual-editor .editor-block-list__block {
max-width: 1200px;
}
/* HTML-block */
.wp-block-html .block-editor-plain-text {
max-height: 700px;
font-size: 14px;
resize: both;
}
Since I am using a child-theme I use get_stylesheet_directory_uri() in functions.php. (get_template_directory_uri() otherwise)
I also wanted to have more space in the HTML-block.
Thanks for answers and suggestions Tom and Juan.
Takes some time to get used to the Gutenberg editor!