Grand(e) can mean "long" when it is being used in reference to some relative dimension. http://www.wordreference.com/fren/grand A "big" dress implies to me something like the kind of dress that Deborah Kerr wore in "The King and I".
That's what I thought as well, then I looked it up. I think that 'grand' is 'great' only when it is referring to a person or to people, not objects, when it's placed before the noun (the person). Can anyone else verify this?
There are exceptions I think: "un grand jour" is "a great day".
However, "a great dress" would not be translated with "grande", more probably with "une belle robe" or something more emphatic, like "merveilleuse" (wonderful), "fabuleuse" (fabulous)...