Items made from acrylic yarn or fibers do not shrink when washed. The items also do not shrink in the dryer, so it is safe to include the pieces with other types of laundry when washing.
If the item is purchased in a retail store, consult the label to determine the content of the fabric. While an item made only of acrylic does not shrink, one that includes other materials, such as cotton, does have the potential to shrink. If the item is handmade and its fiber content is unknown, wash the piece by hand and lay it flat to dry to avoid shrinking or stretching.