Add custom editor font sizes to your site after the block library styles using wp_inline_style.

functions.php

<?php

add_action( 'enqueue_block_assets', 'rgc_editor_font_sizes', 15 );
/**
 * Add custom font sizes to the site using wp_inline_style, attached to the wp-block-library stylesheet.
 * Editor font sizes must be defined in your theme.
 */
function rgc_editor_font_sizes() {
	list( $font_sizes, ) = (array) get_theme_support( 'editor-font-sizes' );
	if ( ! $font_sizes ) {
		return;
	}

	$css = '';
	foreach ( $font_sizes as $size ) {
		$css .= sprintf( '.has-%s-font-size {font-size: %spx;}', $size['slug'], $size['size'] );
	}

	wp_add_inline_style( 'wp-block-library', $css );
}