Q1: What fabric is it?
A1: Polyester. So there is some stretch to it.
A2: Polyester and spandex, soft enough.
Q2: Is it true to size?
A1: I am 5'6" 160 lbs and a medium fit with room to spare.
A2: I got medium, I'm 5'7" 145 lbs, and it fits well, plenty loose.
A3: Small fits perfectly. I'm 5'5" & 114lbs.