I’m not certain about the wattage. However, when I plug in my phone, it displays around 6-7 watts. I believe the cable that came with the speaker was recommended, but will using a charger with a higher wattage harm the speaker?
No it will not harm your speaker. Controller inside speaker will take as much as required to charge speaker.
The inbuilt electronic controller checks if the speaker is full charged and takes only the power from the charger which is needed.
As long as it’s a USB Power Brick you will be fine as the other’s have mentioned.