Is it something women would wear? If so, in a style of their own, or a man's cut? If the latter, you may sell more Sm and Md than if only men are buying men's shirts.
At LOT depends upon the typical "girth" of your target demographic; these days, age isn't a great indicator of that, but other things might be. Are they basement dwelling nerds, exercise freaks, health nuts, fast food eating TV zombies?
If there is really no way to make a good guess, I would go very light on the extremes and heavy on the middle. Something like: 5, 25, 30, 25, 10, 5 if just for men.
Maybe: 15, 30, 30, 20, 5, 0 if 50% women.
I sell everything from Sm junior cut ladies to 5X boxy men's shirts to ... women. One size/style does not fit all. I've noticed many times when looking at listings online, that Sm men's is typically the last size left when a design is mostly sold out--so it seems many over estimate the demand for that size.