To shrink a rayon dress, machine wash and dry it on a moderate-to-high heat setting. Rayon is a very delicate fabric, so if the tag on the dress says "dry clean only," be sure to hand wash it before placing the dress in the dryer.
Some rayon blends can stretch further in the wash, so read the care label before placing the dress in the wash. Dharma Trading Co., a company that specializes in treating and selling fabrics, recommends hand washing rayon to preventing dye bleeds and possibly ruining the dress. After hand washing the dress, it can be put in the dryer on moderate-to-high heat as long as it is dried alone.
Most rayon is not pre-shrunk, but occasional rayon blends are pre-treated to make the fabric more resilient. Rayon shrinks very easily and continues to shrink each time it is washed. For best results, shrink the rayon to the desired size, and then hand wash and line dry it to prevent the dress from getting smaller. It is never advised to wash rayon or rayon blends with other clothing, especially tough fabrics like denim, as these fabrics can bleed or pull at the seams of the rayon and stretch it out, dramatically shortening the life of the dress.