Cotton shirts tend to shrink after repeated washings. They may not fit as well as they used to. Or, you may have some vintage T-shirts that run a little small that you want to expand so you can wear them comfortably. Sizing isn't what it used to be, and a size Large in a vintage T-shirt may be equivalent to a size Medium today. Expand your shirt without ruining it so you can enjoy your cotton T-shirt collection a bit more.
Pour white vinegar into your bathroom sink and plug the stopper. Put your shirt into the vinegar and get it completely saturated. Wring the shirt out.
Place the vinegar soaked T-shirt into a zippered plastic freezer bag for half an hour.
Remove the shirt from the bag and pull it out a little bit to stretch it, then start the washing machine using hot water.
Toss the shirt into the washer and add mild detergent. Wash it as you normally would.
Dry the shirt in the dryer, using high, hot heat for an hour. Remove the shirt from the dryer. This should stretch your shirt at least one size larger.
Wash the shirt as you normally would. This method requires that you work with a wet T-shirt.
Lay the shirt flat on an ironing board.
Iron the shirt, using a low to medium setting, going vertically and horizontally. Pull the fabric both ways to stretch it as you iron.
Hang the shirt and allow it to air dry.