agmtb1636895 wrote an answer about Ibex Woolies Zip T-Neck Shirt - Men's on January 3, 2014
I'm 6', 146 and wear a medium but have small in I/O Bio and Patagonia lighter weight zip shirts. All three have long enough sleeves for my 6' wingspan. I use the lighter ones cycling mostly.
You should be fine in size small.