I am 5'5", 130 lbs, 27" waist, and I have an XS. Based on my measurements, I could wear an S, but it felt too small when I tried it on.
I kind of hate how this harness sits on me, but I think I'm an anomaly. Most people seem to love it. I can't get it to chinch up as tight as I want it, and when it's weighted down with gear the quick adjust slips and makes me need to readjust it constantly, which is an ongoing struggle.
Also, in such a small harness, it feels like everything is squished together, making it just awkward to cinch up and adjust properly. I can't really blame Misty Mountain for that, but I do feel like I'm on a never-ending mission to find a harness that fits a small woman (I'm not even that tiny!!) and can carry gear comfortably.
When I called Misty Mountain to discuss this, they said that the fit was fine (and that an S would have been fine too), and they offered to send me a non-quick adjust version, which I appreciated but declined.
As far as the loops, I purchased this harness specifically because I was struggling with fit in other harnesses and also racking up gear on four loops. I find the six loops on this a bit of overkill, and I'm still unable to find a way to manage how I organize gear on so many loops. I find that racking anything on the lower loops on this makes my gear hang a lot lower than I find comfortable for climbing, so I avoid it.
Hanging in this harness is also quite uncomfortable, unlike what I read in other reviews. I feel like this pinches nerves in my legs in a weird way, and I can barely stand hanging in it longer than a long lower, let alone a hanging belay.
So, with that said, if anyone knows of anything better...please share!