Polyester is one of the hardest materials to shrink, according to Made Man. This is an advantage if a polyester shirt fits well, but it presents a challenge in getting an oversized shirt to fit.
Wash in hot water
Set the washing machine to the hottest possible water temperature setting. Polyester shrinks best at the hottest possible temperature, says Made Man.
Dry with hot air
Dry the shirt on the dryer's hottest setting. The combination of the hot, wet shirt and the hot, dry air contribute to shrinkage.
Check and repeat
Try the shirt on. If it fits, the process is complete. If it is still too large, repeat the washing and drying cycles, and try it on again.