Safest way to shrink a cotton t-shirt?

I bought a really lovely t-shirt online, in a size M however it fits rather like an XL!

Instead of having to actually alter it, I was hoping there is a safe way for me to shrink it. The shirt has quite a delicate, glossy print on it – which is why I am particularly uneasy about washing and drying on high heat. It is made out of 100% cotton, which is why I thought about possibly shrinking it, even slightly. The washing instructions do suggest that simple machine wash is acceptable, however.

Any suggestions would be appreciated!