All Apple Watches will use ONLY what they need. The Series 7 will use no more than 20watts during fast charging (the up to 50% point), then it will use less, no matter how much potential the charging brick has to offer.
This applies to all Apple Watches. They will use ONLY what they want, and will not care if the source has more to give. The source cannot force more power into the Apple Watch. The Apple Watch regulates how much power it takes, and stops taking when it is fully charged.
So you can even use the 140watt M1 Max MacBook Pro charger, and it will not matter and it will not speed things up any more than a 20watt charger will.