A New York based company, threadbase, has done some extensive research and testing on t shirts. They’ve analyzed by washing, drying, measuring, and weighing a whopping 800 men's t shirts.
This compilation of data shows a comparison of shirts and their specs side by side, a small Hanes Beefy T is the same length as a medium Fruit of the Loom Crew Neck or a large Eddie Bauer Classic Fit Legend Wash has the same chest size as extra large American Apparel Jersey. This is a great illustration of how different shirts can be.
Also. threadbase tracked the wear of washing and drying the different shirts and how this affects their fit. For people who are really into t shirts, like us, this data is fascinating.