What adapter size do I use for Apple Watch 6

If you are talking about the USB power brick, then any USB plug you can find. It can be a USB port on your personal computer, it can be a USB power brick for your iPhone or iPad. Basically any USB port that has the correct USB plug to match your charging cord.

If you are referring to band sizes here is the information that you need.

38mm and 40mm watch faces use the same band

42mm and 44mm use the same band

What Size Watch Band Does My Watch Band Take

In the Watch app on your iPhone go to My Watch, then at the top of the page tap All Watches. You will see the entry for your watch, including the case size. This tells you which band size you need to order, for 38/40mm or 40/44 mm case.