Big Picture in a bedroom with low ceilings
When you say big and low? What exactly do you mean.
Generally, you hang art so the middle or focal point of the piece is a few inches around your eye level (depending on height although museums have a specific formula). The ceiling shouldn't make a lot of difference unless the piece is so big you have to shift that focal point too far up or down. Of course if the pieces is so tall you have no wall or "white space" around it, the presentation will look very cramped.
If the ceiling really draws attention to itself think about painting it something other than white---especially if you have color on the walls.