In the Witcher 3 when Geralt get's a haircut to something shorter, does the hair grow back like the beard? For example if I get the long top with the pony tail but shaved sides? Will the hair grow back to long again?
As others have mentioned the facial hair will grow back only if you only choose either the clean shaven or trim beard options. If you ever choose one of the other DLC facial hair options then your facial hair stops growing.
As far as the hair on your head that doesn't grow with one exception. If you end up being thrown in jail in Toussaint your hair cut will change to the 'long and loose' hair cut. That is the only time I've seen your hair cut change or grow after styling it (assuming of course your hair wasn't styled that way prior to going to jail).