I have the same projector, the same viewing distance and I went with a 96"x54" screen. I am only using the middle 4:3 section of the screen at the moment and I think the size is perfect, the pixels are just visible. Besides, 55" wide screen material is readily available for about $5/yard.
I also sit about the same distance and I am projecting on a 50"x67" (84") screen, and also using the same projector LT154. I think the picture looks great, and the size is just perfect for the room size.