Sweater dresses are particularly tricky for the tall girl. Length, as always, is crucial to keep one from looking like a streetwalker. If you go too large, it can make you look exponentially massive, speaking from experience. Should the size be too small, it can make you look like a streetwalker, even if the length is right. Sweater dresses tend to be clingy to the point of bordering on body con.
Pro tip: Shop at online retailers that offer free returns or returns to brick and mortar and make sure that there is a location nearby before dropping money on two sizes. Return the one that does not fit. Return them both if neither fits and pick another target.