im 5'9" and weigh about 135, im pretty...
im 5'9" and weigh about 135, im pretty skinny, i usually wear a medium in normal shirts which are usually 38"-40", and thats even a little too wide for me, these say they're fitted which is good but the analog sizing chart on this page says a small ranges from 36"-38" and a medium 38"-40", but does that mean for these fitted shirts or just for any analog shirt?
I wouldn't put too much stock in our size chart, but I would go with a medium if I were you. That way you can toss em in the dryer and not have to worry about them shrinking up too small.
I agree Medium should fit perfect. Your going to love these..