I'm 5'8"/143lb, size M or S ???
BRIAN R I'd go with a medium. These jackets are form fitting and a small will not be comfortable with anything underneath.
MEG M If it helps I am 6' /165 lbs and bought a Large. (This was about 2 years ago though- so I'm not sure if the cut has changed) The fit is good for me all around- but I would have liked about 1 inch longer in the arms. (i have long arms though) It fits pretty trim. It has been a really good jacket, I'v e used it lots and it still beads water. Fabric is pretty durable.
JONATHAN C Medium for sure. A small would be tight if you had anything else under the shell
JUSTIN N Mammut runs VERY small. Like Patagonia slim fit on steroids small. I'd lean towards medium. On the flip side you probably won't be wearing a ton of layers under it, usually I just have a t shirt or base layer under.
It's a great jacket. The euro zipper takes getting used to. Hood is awesome, I hate hoods that don't adjust properly, this one does.