I did some research into garmet sizing
To the left of the average, there is a point known as the 5th percentile, because 5% of the people (or 1 person in 20) is shorter than this particular height. The same distance to the right is a point known as the 95th percentile, where only 1 person in 20 is taller than this height.
There are 4 main anthropometric measurements. These include weight, height, body mass index (BMI), body circumference (arm, waist, hip and calf), waist to hip ratio (WHR), elbow amplitude and knee-heel length.
For the moment, I will design to suit a Medium adult, and make it adjustable to be smaller or larger, to suit the majority. Just for the moment anyway.