I make quilts for my beds as well as for wall hangings. I generally only use Warm and Natural cotton batting which is pretty thin. I can fit a King Size in my washing machine which is one of those enormous front-loaders. If you've got a regular top-loader a King Size might fit depending on the size of the machine, but if you're short like me (5'2") you'll have a hard time getting it out of the machine. That's why when my last machine broke I went with a front-loader. None of my quilts are bedspread size, I make them so they come down over the sides of the mattress, but they don't cover the box spring, but you can do whatever size you prefer, there are no hard and fast rules.
Rob