Is the TV on the wall at the foot of the bed, or on the side? How far from eyeballs to screen?
I currently have a 32" in a smaller bedroom (12x13) but have used a 50" plasma in a 12x14' bedroom in the past. IMHO, it is not too much. The plasma will add heat to the room, so it kind of depends on whether the room already runs hot. Those 17' ceilings should act as a heat capacitor by allowing the hotter air to rise away from you. The Sony won't generate a lot of heat.
As for size, I have a friend who projects a 92" image onto the foot of his bed. You get used to the large size quickly and never want to go back. IMHO, 55" is just fine.