Fill a clean sink or plastic tub with cool water. Not hot water, which will tighten the fibers even more, and not cold water, as the conditioner won't thoroughly dissolve. A few drops of water on the inside if your arm should be cool, but shouldn't make you shiver.
Pour 1/4 cup of hair conditioner into the sink and agitate the water with your hands to break up big globs of conditioner. Any plain, cheap conditioner will do, so don't waste expensive salon conditioners on shrunken shirts. The conditioner softens the fibers and allows you to stretch the shirt back into shape without causing permanent damage.
Lay the shirt over the surface of the water, and push it down evenly with your hands to submerge. Wadding the shirt up won't allow the conditioner to penetrate all the fibers, so push it down as flat as possible into the water. Let the shirt soak in the conditioner-infused water for 15 minutes.
Drain the conditioner water, and fill the tub with clean, cool water. Squeeze the shirt gently to force out conditioner water, and soak for another five minutes. Repeat the clean water infusion until no traces of conditioner remain.
Squish as much water out of the sweater as possible, and lay it on a stack of clean towels. Set one hand on the neckline of the shirt, and use the other hand to gently stretch the bottom hem down. Work from left to right, pulling the hem down an inch or two to resize the length of the shirt.
Grab one side of the shirt in each hand, and pull the shirt edges in opposite directions. Gently tug on the shirt to widen it, keeping your hands level with each other to stretch the shirt evenly. If the sleeves are too short, pull each sleeve cuff down and slightly to the side to stretch the sleeve in a natural direction. Leave the stretched shirt on the towel to air dry and retain its new, larger shape.