Snippet #009 Custom Gutenberg font sizes https://since1979.dev/wp-snippet-002-changing-the-gutenberg-font-sizes/

change_gutenberg_font_sizes.php

<?php
/**
 * set_editor_font_sizes.
 *
 * Add theme support for editor-font-sizes,
 * and set font-sizes for the gutenberg editor.
 *
 * @see https://since1979.dev/wp-snippet-002-changing-the-gutenberg-font-sizes/
 *
 * @uses add_theme_support() https://developer.wordpress.org/reference/functions/add_theme_support/
 * @uses __() https://developer.wordpress.org/reference/functions/__/
 * @uses array() https://www.php.net/manual/en/function.array.php
 * @return void
 */
function set_editor_font_sizes()
{
    add_theme_support('editor-font-sizes', array(
        array(
            'name' => __('Small', 'your-textdomain'),
            'size' => 12,
            'slug' => 'small'
        ),
        array(
            'name' => __('Medium', 'your-textdomain'),
            'size' => 16,
            'slug' => 'medium'
        ),
        array(
            'name' => __('Large', 'your-textdomain'),
            'size' => 24,
            'slug' => 'large'
        ),
        array(
            'name' => __('Bonkers', 'your-textdomain'),
            'size' => 40,
            'slug' => 'bonkers'
        )
    ));
}

/**
 * Hook: after_setup_theme.
 *
 * @uses after_setup_theme https://developer.wordpress.org/reference/hooks/after_setup_theme/
 * @uses add_action() https://developer.wordpress.org/reference/functions/add_action/
 */
add_action('after_setup_theme', 'set_editor_font_sizes');
?>