Bed is against the wall and rug will not be centered.

It depends. What is the room size? How much rug do you want showing? Is it for looks, warmth, etc? What is the flooring? What is your decor style?

You can use just about any size you like.

It would help to show the layout of the room. And with photos, mock-ups could be done to give you an idea.

I agree, you can use any size, since no one knows what the room looks like.