For the most versatility, the little black dress you choose shouldn’t be too short. It should be above the knee, but in an appropriate length for business as well as stylish enough for evening wear. Since these dresses are meant to be elegant, too short a length would defeat the purpose.
The fit of your season-spanning black dress is important. An A-line shape is probably best if you tend to fluctuate between dress sizes. You could always use a belt to add waist definition. A wide belt flatters most figures as well as little black dresses, but if you’re slim you could also choose narrow or chain belts. The best fit for a little black dress skims your body rather than clings to it or hangs too loosely. (source from:wisegeek.com)