You wont be able to shrink it 2 sizes smaller but try to wash it in warm water and dry it in a dryer with warm air. That usually (accidentally) shrinks my stuff. First however check the tag and see if it is preshrunk. If so there is nothing you can do.
Cotton naturally shrinks when you wash and dry it. However, if it is preshrunk, it won't. Check the tag to see if it is. If it is, it won't shrink any more.
