Traditional cocktail dress code etiquette states that a dress’s length is to the knee, but nowadays you can go for something a little shorter if you wish. Short, strapless styles can look great on young ladies. I typically wear at the knee or petite, above the knee.
Just remember to keep it dressy and elegant, but not too formal. The appropriateness tends to be judged more on its sumptuousness rather than length so you should lean towards sensual fabrics like silk and crepe in good strong colors.
From my closet I wore the above items to a resent cocktail party. Here are some general notes I found on the internet. Source: Etiquette and Manners.
The Occasion: A Cocktail Party
Old etiquette: No surprise here―a cocktail dress.
New etiquette: Cocktail dresses are always in style - There are other options.
What to wear: These days, a cocktail party can be anything from a swanky society affair―cue that glittery knee-length number from the special occasions department―to a low-key group of friends gathered around a platter of canapes. But for the most part, cocktail parties are dressy-casual, so you can’t go wrong if you wear a fitted jersey dress (like the one shown) plus heels or fancy flats. Avoid fabrics that are too casual, like chino, jersey, and denim. Also avoid over sexy and remember this is business.