When contemplating a tee shirt for a promotion, or as advertising, or for your line of art that you hope will make you a household name, you have to understand that the shirt its self says more about you than you realize.
With a 100% cotton shirt you have a variety of yarn processes, from carded cotton, which is rough, to ring spun, which will produce a much softer garment. One is much more durable and the other is much more comfortable.
Of course, now you also have tee shirt manufacturers that take a courser cotton and treat the fabric with silicon to give it a softer hand. It also can lead to problems with screen printing.
In regards to screen printing you will also find that the thicker and courser the yarn the better the ink holds, the smoother the ink sits on the shirt, and the longer the design will last. The best shirts for screen printing are those from 18 singles yarn. Once you start getting to 24 singles and above you are basically screen printing a "throw away" shirt.
If I was putting my company name on a shirt, or promoting a cause that I believed in I would want a shirt that was going to stand the test of time.
Remember, the graphics may dazzle but if the shirt is not comfortable it will not be worn. If you are doing a promotion, then you need to realize that you get additional mileage from your promotion the more people wear the shirt.
If you have no clue as to what sizes you will need, then the safe bet is a 1-2-2-1, or 1 M, 2 L, 3 XL, and 1 XXL.
Oh, and always design your graphics with the color of the shirt defined! Never design on white unless you are going to print on white! One other key point, a design may really "pop" on your computer screen, but you need to realize that you need the design to "pop" when someone is standing 8 to 12 feet away from someone wearing your shirt!
One last point, if your market is teenagers then slim fit and 30 singles are great shirts but once your customer reaches 26 or 27 years old, slim fit and 30 singles yarn just isn't going to be all that attractive as a shirt any longer!
Here is a great article on T Shirts, Venture Capitalists, and Start Ups!