The general rule for ensuring your bed will fit in your room is to give yourself about two feet of space around three sides of the bed. So let’s say you have a California king, which is six feet wide and seven feet long. That would mean you need a bedroom that is absolutely no less than 10 feet wide and nine feet long to give you the requisite two feet on every side.
However, you also have to think about bedroom furniture. If you want a dresser, armoire, or other storage, you’ll need to have at least two more feet at the foot of the bed or on one side. So for your Cal king bed and a dresser, you would need a room of at least 10 by 11 feet. You will probably need even more space if you want bedroom seating or a vanity.