Cocktail dresses are the most popular choices when you are invited to attend a party. It is semi-formal style. Also, the cocktail dress (http://www.1st-dress.com/Cocktail-Dresses_v157/) is simple and elegant. Normally the cocktail dresses are short style, but if you do not want short style, then you could choose long style.
The short style cocktail dress looks lovely and cute. But with the change of fashion trend, these cocktail dresses now are available in various designs, styles, fabrics, shapes and lengths. Speaking for the length of the dresses, the common types of them is above the knee. Besides, there are types that would run low, down to 2 inches above the ankle.
By the way,please remember, you should avoid wearing a sequined gown which is just too formal to wear in such a casual occasion.