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.
The below sweater dresses are available in tall. My pick is the gray with the pearl embellishments and the gray fair isle.