Enable and set plupload scaling dimensions

plupload_scaling.php

function plupload_scaling( $defaults ) {
	$defaults['resize'] = array(
		'width' => 1024,
		'height' => 1024,
		'quality' => 100
	);
	return $defaults;
}

add_filter( 'plupload_default_settings', 'plupload_scaling' );