The Seleucid Empire is another country created after the death of Alexander the Great. It's one of the largest countries you can manage at the beginning of the game in Imperator: Rome.
The Great Seleucid Empire really is great and has a vast area. On the one hand, you have a lot of resources and rich territories at your disposal. However, you will have to face many internal problems as well as Egypt, Macedonia or the great kingdoms of India. It's not easy to maintain such a big state. Playing Seleucids is a big challenge, especially at the beginning of the game.