First off, let me say there is no hard and fast rule for this.
Depends on your target audience.
A few years ago I was told that, as a starting point, the "normal" size breakdown for adults is approx
3%Small, 13%Medium, 39%Large, 36%X-Large and 9%XXLarge.
This will provide a pretty good base from which to tweak the size assortment based on your target audience.
If you intended audience will have lots of women or people that are young and physically fit...
you may want to skew the sizes slightly smaller than the norm.
If on the other hand, you will have more men, older ...or sturdy types (your customer is a group of construction/quarry/... workers),
then you' may want to skew the sizes up the XXL end of the scale...and forget small entirely.
As I said at the start, it's a guide, not a science.