The key to the project is to make the pedestal sturdy, especially for front-loading washers, since they have a tendency to move around if they're not on a solid, level surface.
To start, measure the length and width of your washer and dryer. That's your minimum size for the pedestals. You can make them longer and wider if you want the extra space to stand on to reach overhead shelving or to set the laundry baskets on. Or you can make one long pedestal to fit under both the washer and the dryer (see illustration).
Build the pedestal out of 2x10 or 2x12 lumber. Feel free to add an attractive finish. Use shims to level the pedestal before you set the appliance on it.