Upload max filesize | WP Multisite

upload_max_filesize_wpmu.php

/**
 * @snippet      Upload max filesize | WP Multisite
 * @comment      Put the correct blog id number in the if condition. Currently set to the main blog.
 * @source       https://kybernaut.cz/clanky/wp-multisite-maximalni-velikost-nahravaneho-souboru/
 * @author       Karolína Vyskočilová (https://kybernaut.cz)
 * @testedwith   WordPress Multisite 4.9
 */
// -------------------

function custom_filter_fileupload_maxk( $maxk ) {

	global $current_blog;

	if ( $current_blog == 1 ) ) { // number = blog id
		$maxk = 5000; // allowed size in kb
	}

  return $maxk;
	
}
add_filter( 'pre_site_option_fileupload_maxk', 'custom_filter_fileupload_maxk' );