When deciding what size to order, it's always recommended to measure an existing shirt you have to compare measurements (remember though, cotton shrinks!).
If you look at the size charts online, you will see a measurement for W and one for L. W stands for width or chest.
You measure this by measuring across the body of the shirt (laid flat), typically from armpit to armpit.
L stands for length, measured from the highest point on the collar, OR center back collar, down to the bottom hem.
S is a measurement seen only on long-sleeve shirts. This measurement is taken from the center back to the bottom of the sleeve.