I bought the Medium strap size and the Small strap size, just in case. I read other reviews saying that the S size was very small. On the contrary, S, for me is too big and M is very large. I use the M around my hips and the S around the top of my waist. There are measurements on the sales page - just buy the one for your measurements.
I love the feel of the material - really smooth and when used, you can't feel that you're wearing it. My mic pack fits very snuggly into the pocket so there's no chance it'll fall out.
The thing I love the most for this is that no matter the size, you can use the velcro anywhere on the strap so if the M is too long, you can still close it with the velcro.
Top purchase!