There is some info here http://luma-labs.com/products/cinch. Scroll down almost at the end of the page.
Easy Sizing and Fit
We know that photographers come in all shapes and sizes and we want everyone to be able to use a Cinch. So we make Cinch in three sizes. Most people will be comfortable with a Regular cinch. If you're on the small or big side, then you should get the small or big Cinch.
Put another way, if you wear an adult M, L, or XL shirt (either male or female), our regular Cinch will do the job just fine. If you’re smaller than that, you should probably get a Small Cinch. If you wear XL or larger shirts, then get a Big Cinch. But don’t stress out too much about your choice. We've given the sizes a considerable amount of overlap so picking the right size is easy to do.
Once you have it in hand, fitting Cinch is easy. Just put your Cinch over your shoulder, loosen the pull tab adjuster and the use the rear metal adjuster until your camera is positioned just right. If needed, you can also tighten or loosen the 10mm webbing that connects to your camera to adjust things to perfection. Once set up, the front pull tab adjuster gives you just the right amount of on-the-fly adjustments to keep up with any day of photography.
Gear is important but skills are very important