A room that size with a piano?? Go with headphones or some PC Audio type nearfield active monitors. If going with monitors, get a capable micro sub like the KEF KC62 to go with it.
Edit:
Get the KEF LS50 Actives. The concentric driver should do well in nearfield applications. KEF has always saved the day for guys with closet sized rooms. You will now also have enough room to adequately treat your room (both your piano and your speakers will sound a lot better with treatment).