What are the right volts and watts to charge my iPhone 6s Plus?

Always 5V as that is USB standard voltage. The wattage should be at least 5 as an iPhone power brick puts out 5V at 1 Amp. But since the device governs it’s own charging, if your power supply has higher current it won’t matter, since the iPhone itself will only draw as much current as it’s designed to.

You can, for example charge any iPhone with any iPad power supply. I use 12Watt iPad power bricks pretty much all the time for my iPads and iPhone since I have several of those 12Watt iPad power converters around.

5volt

https://support.apple.com/en-us/HT202105