While dresses tend to be the default for most holiday soirées, they are certainly not the only way to go. Separates can be a great option. Opting for a great skirt, blouse, etc. gives you a bit more flexibility with your look and, depending on what you choose, you may be able to get a lot more wear out of a separate piece than a dress.
Below, I've put together some amazing plus-size options that would be great for the holidays and can be worn long after.
P.S. The blouse and skirt in the first picture are available at Eloquii (here and here).