Does smoking pot make a guys dick longer?

yes because since it affects blood circulation around your body, so in theory will affect blood in penis. A penis is kept erect by chambers of blood around, in theory would mean a decline in Penis size.

ALSO:

It may well reduce your libido (Desire for sex), and will reduce you physical stamina resulting in a sex life that is not as satisfying as is could be (you get short of breath so have to "Finish" when you would have rather carried on a bit longer!). (Ex smoker talking)

It can also cause impotency due to reduced blood flow.