The first thing you should consider is the purpose of the humidifier. From here, you can move on to how large the appliance is and how far it should be from the bed. You should also consider the number of people in the room as well as the size of the room. With these factors in mind, placement will not be much of a hassle.
One excellent place to have the humidifier is towards a wall on a stable surface that you cannot reach in your sleep. In this way, the appliance would work without toppling over or spreading germs to you as you sleep. In such a position, if anything were to make the gadget fall, you would be safe.