When it comes to black-tie attire, its not so simple for women. A black-tie invitation can be frustratingly vague. We can't simply put on a tux and go. There are so many decisions to make, short, long, evening gown, silk, satin, velvet....
Traditionally, long is more formal than short. However, the choice is based more on what looks best on you. Factor in age, legs, figure, and comfort level. A long dress is best if you can't wear heels. A short dress is always a good choice if you are unsure of how dressy the event will be.
Remember, black tie doesn't mean you have to wear black, although to play it safe, opt for a simple black dress, the best canvas for statement jewelry
The most important factor, be comfortable and have a good time.