if the room is 12x12 and you put a 6' bed in it, that leaves 3' each side which is a bit cramped ! say the bed is 7' long and you have a 2' deep piece of furniture either on the bottom wall or at the end of bed - that leaves only 3' gap at the base too . and if you buy smaller furniture it's just going to make the bed look even bigger !
so keep everything as light coloured as possible - add a few mirrored prices if you can as they help bounce the light around too ! and if you can bear it - choose a smaller bed ?!?

Do you have a photo of the room? A king size bed measures about 7' x 7' which leaves little room for anything else. What are you wanting or more specifically needing to put in this bedroom?