[WordPress][WP Rocket] As of 2.11, WP Rocket generates critical CSS automatically. Here’s how to adjust parameters for maximum viewport width and height.

wp-rocket-critical-css-params.php

<?php
/**
 * Customise parameters for critical CSS generator.
 */
add_filter( 'rocket_cpcss_job_request', function ( $item ) {
	return array(
		'url' => $item['url'],
		'mobile'          => 0,   // 1 = Emulate a mobile device
		'viewport_width'  => 1300 // px maximum width of the viewport
		'viewport_height' => 900  // px maximum height of the viewport
	);
}, 10, 2 );