I love my 686 Smartys. I've had a couple pairs of 686 pants (Smarty and some of their limited edition) and I've worn Nomis (tore those up quick), Nike and DC. I am 5'7" 140# about 31" inseam and I rock a M for a little baggier fit. They come in tall, regular and short lengths though I've rarely seen the short. The M are definitely long inseam, probably would be ok for someone with a 33" or 34" inseam .
The S will do you fine if you get them T. If you are that skinny and you want somewhat fitted the S will be good. If you want baggier, go M and don't get T, get the regular length.
The only thing these pants aren't good for is late spring riding. When the temps go much above 60F, they are just too heavy.