Open stage curtains only halfway to create a smaller playing area. If there is some stage surface in front of the curtains, consider leaving the curtains closed to create a narrow strip of playing area close to the audience.
Drop a scrim from the rafters of the theater to the stage floor to block off some of the upstage area. Wheel divider panels onto the stage to create a similar effect.
Set your stage lights so they illuminate only a small segment of the stage. Using the barn doors on your lights, angle them so the light is focused on just the area you want to use.
Place floor lights and audio monitors on the stage floor. They are normally placed flush with the front edge of the stage, but you can create a smaller playing area by placing them as far upstage as possible. Arrange them in a semicircle curving away from the audience, and use the area within the semicircle for your performance.