A cotton polo shirt can be shrunk during the normal washing and drying process. These steps will work as long as the shirt is 100 per cent cotton. Make sure you really want to shrink the shirt before taking any action as there's no way to reverse the process.
Put the cotton polo shirt in the washing machine by itself.
Turn the heat on the washing machine to the highest setting. The hotter the water, the more the shirt will shrink. Cotton shrinks most during its first washing, while other fabrics can take multiple washes to shrink.
Add washing powder to the washing machine, shut the door and allow the washing machine to complete its cycle. When the washing cycle is completed, remove the shirt from the washing machine.
Put the shirt in the tumble dryer or on a dryer setting if you have a combined washer-dryer. Make sure the dryer is on the highest possible heat setting. Some shrinkage occurs during the drying process. Allow the dryer to continue for longer than necessary, to the point that the shirt is very hot when removed from the dryer.
Put the polo shirt on and check to see if it is the desired size yet. If it is not, put it in the dryer again. Excessive drying can lead to shrinkage.