While there didn't seem to be any material that outranked the rest in terms of reflective and absorptive characteristics, decorated plaster and wood offered the best opportunity to affect the quality of the ambiance.
In terms of audience seating, one would want enought seats for a number of people to enjoy the performance, but not enough to significantly dampen the sound of the harpsichord.
In terms of the size of the room, anything larger than about 50 feet would negatively affect the projection of the sound volume so that a listener would no longer be able to hear the music as well. The ideal size would be about 35 feet square.
Over this would be some type of dome to evenly disperse the sound waves coming from the harpsichord to the back of the room, where without a dome the sound wouldn't project to fully.
The room should be appointed with Louis XIV furniture and Renaissance paintings so that listeners would be transported into the world of baroque music.