Is it possible to unshrink a shirt?
Answers: It is possible to stretch out a shirt, if it is made of natural fibers (polyester will resist shrinking and will also resist human being stretched.)
I keep a spray bottle of 1/3 cloth softener and 2/3 water handy surrounded by case I overdry my jeans. I freshly spritz on the mixture and wear them around the house until they stretch a bit.
(Works best if you have a garment that have some cotton in it.)
Depends on what nice of shirt. If it's a cotton or synth knit, I'd wash, surrounded by cold water, next reshape, preferably on a thick towel, on a drying rack (don't evacuate on any wood surface) and stretch out evenly, pulling sleeves length & width considerately to stretch a little bigger, and consequently also pull lenght of shirt, while holding shoulder seam to save it even. as it drys you can stretch a little more, hopefully it will regain some of productive size.