Hassan should cut it slightly less than what is intended so around 16 inches or so.
Explanation:
The hair that is washed with shampoo is usually more voluminous looking and fluffy and it goes back to normal in about an hour or two.
This is something that barbers would keep in mind whenever they are styling someone's hair and only work in accordance with that.
As a similar thing has happened here it is the only natural way that Hassan would wash it and then cut it a little bigger than what is needed so when the hair settles later it is as much as Sandy needed.