add-to-cart.php

if ( ! is_shop() && ! is_product_taxonomy() ) {
	$quantity_field = woocommerce_quantity_input( array(
		'input_name'  => 'product_id',
		'input_value' => ! empty( $product->cart_item['quantity'] ) ? $product->cart_item['quantity'] : 0,
		'max_value'   => $product->backorders_allowed() ? '' : $product->get_stock_quantity(),
		'min_value'   => 0,
	), $product, false );

	$quantity_field = str_replace( array( '<div class="quantity">', "</div>" ), '', $quantity_field );
	echo str_replace( '<input ', '<input style="max-width: 70px" ', $quantity_field );
}