Bigger is better when it comes to dorms in my opinion. High ceilings make a room feel more spacious, which is especially important when you are sharing a room with however many people.
I'd say you would be fine to use it as a dorm for now until you divide it into 2 floors.
Are you going to have issues with the room being cold though? With such a high ceiling, all the heat will rise and the room could feel very chilly.