The upstage areas are lighted from a bridge or pipe mounted directly behind the curtain. The usual layout calls for six spots covering the downstage section and six spots covering upstage. Footlights should not be used to light the actor. They create unnatural shadows. Color is usually achieved with sheets of colored gelatin placed in a frame in front of the instrument.
Intensity is controlled by dimmers.
With dimmers the stage can be given any light from blackness up to bright intensity. A popular dimmer is of the resistance coil type, similar to the volume control on the first televisions. More flexible are the autotransformer, the reactance, and the electronic dimmer board. So, to answer the question. The playing space, surrounded by the audience, may be a circle 15 or 20 feet in diameter or a square rectangle of comparable size.