I had this same discussion maybe a week ago with some friends! As it was explained to me, gros/grosse is used for big as in horizontal largeness while grand/grande is used for vertical largeness. When applying the two adjectives to objects (dresses and the like) use the one that would generally depict the way that something would normally be enlarged.
That said, people can be either from what I understand...but DuoLingo has told me I'm wrong before when I use gros/grosse in reference to a person in a sentence.
...It's a real head scratcher.