Snippet to change Label size(LabelStockType) in FedEx create shipment request. WooCommerce FedEx Shipping Plugin with Print Label - https://www.pluginhive.com/product/woocommerce-fedex-shipping-plugin-with-print-label/

functions.php

/**
 * Snippet to change Label size(LabelStockType) in fedex create shipment request.
 * Created at : 08 Nov 2018
 * Updated at : 08 Nov 2018
 * PluginHive Plugins : https://www.pluginhive.com/plugins/
 * Gist Link : https://gist.github.com/varun-pluginhive/4ca11475002212e4f6930c0c6ca83d44
 */
add_filter( 'wf_fedex_request', function( $request ){

	// Fedex Services for which the label size need to be changed
	$services = array(
		'FEDEX_1_DAY_FREIGHT',
		'FEDEX_2_DAY_FREIGHT',
		'FEDEX_3_DAY_FREIGHT',
		'INTERNATIONAL_ECONOMY_FREIGHT',
		'INTERNATIONAL_PRIORITY_FREIGHT',
		'FEDEX_FIRST_FREIGHT',
		'FEDEX_FREIGHT_ECONOMY',
		'FEDEX_FREIGHT_PRIORITY',
		'FEDEX_NEXT_DAY_FREIGHT'
	);

	$LabelStockType = 'PAPER_LETTER';			// Label Size

	if( ! empty($request['RequestedShipment']['ServiceType']) && in_array( $request['RequestedShipment']['ServiceType'], $services) ){
		if( ! empty($request['RequestedShipment']['LabelSpecification']['LabelStockType']) )
			$request['RequestedShipment']['LabelSpecification']['LabelStockType'] = $LabelStockType;
	}
	
	return $request;
});