In addition to those 6 inches of space in the back, you should also leave about an inch of clearance on each side of the washer. Without it, the machine might bump up against the dryer or the wall... especially if yours tends to walk across the floor. Because there's no universal standard washer size across the major washing machine brands, dimensions are going vary tremendously depending on whether it's a front-load, top-load, compact, stackable, or large capacity machine. It's easy to forget, but the machine is going to have to both fit through the door, and also have enough space around the machine for the door to swing open all the way. You'll need at least 20 inches of open space in front of or above the appliance as well as an unobstructed path through whatever doorways, hallways, or tight corners you'll need to get through upon delivery.