Obviously, you cannot expect a small dehumidifier with a miniature reservoir to impact the humidity level of a giant room. Make a logical choice, since the size of the dehumidifier, and the size of its reservoir, goes hand-in-hand with the size of the space in question.
Your dehumidifier's location is important to the air flow. In a room with ceiling ventilation, such as an air duct, position the dehumidifier against a wall. Your dehumidifier's location is most important in rooms without any air ventilation. In such rooms, position your dehumidifier near the center of the room.
Clean the room before you use the dehumidifier so you do not get any unexpected junk in the reservoir. To keep your machine low-maintenance, make sure the surrounding area is kept spotless. This includes removing any mold or mildew that appears around the room.