The size of the chair is irrelevant. According to feng shui theory, it is better to lean against the wall behind the chair, which means that there is a backer. If there is a mirror or water behind the chair, it is very taboo. If this happens, it is best to adjust the chair in time.
In home geomantic omen, it is very taboo to have a cross beam coping. There can be not only a beam above the bed, but also a beam above the chair. If there is a beam above the chair, its fortune will be suppressed by the beam.