I know what you mean, you mean the opposing forces in different books, movies, and games. Whether they're a villain or a supernatural force, they're still an antagonist and they're a dark power in whatever it is.
If we do that we're gonna shorten it to either antagonists or villains. :3
Generally, we try to stay around the ZU limit, which is 650 width and 240 height. However, sometimes people get creative and make things that are a bit larger, or maybe even like vertical. So it's okay to fudge it if you need :3