iphone 6s maximum charger watt

You can use it, however the iPhone will only use what it needs to charge. The iPhone 6s is not capable of fast charging, so it really won't charge any faster than it would with the default 5w power adapter.

The iPhone's "charger" is actually built-in the phone itself. It's this charger that manages the level of charge required coming from the external power adapter. It will not pull more power from the adapter than is required, regardless of the wattage rating of the adapter.

The advice that Tesserax provided is accurate. It will not damage the device.