just because a shirt label says small, medium or large doesn’t mean it would be a perfect fit. the tag on the shirt that indicates if it’s a slim fit shirt, a relaxed fit or a classic shirt also says a lot about how the shirt would look on you.
The Extra Slim
It would be perfect for you only if you have slim and athletic body type with a wide chest , narrow waist and a narrow hips
The Slim Fit
Its the normal fit for you, if you are not slim but not athletic either, if you are the type that doesnt have a perfect 6 packs but you dont have much belly either.
The Relaxed fit
If you are slim and athletic, broad chested and with a bit belly or maybe you just dont like you cloths too snug on you, or you like having extra room is you shirt to breath in then the relaxed fit would be perfect you.
The classic Fit
If you have more than a bit of extra flesh around your waist or belly then the classic fit is a more perfect and obvious choice for you, it has extra space to accommodate the extras you have on you.