Change the thickness of the yarn or the size of your hook: thin yarns and smaller hooks make a piece smaller, thick yarns and larger hooks make it larger.
How I do it is as I am making it I look at the size of it often and I add and decrease stitches according to how it looks if it looks big decrease if it looks small increase but always remember do your increases and decreases a little at a time so they wont show in the finished product.