Nice Vintage Condition! This is a vintage medium, which is more like a modern small. Please see the measurements. The material is thin and slightly sheer from age/use. Material is piled in some areas. Please see the photos and measurements below.

When laying flat, armpit to armpit measures 20.5 inches. 
When laying flat, shoulder seam to the bottom of the arm-sleeve measures 10 inches. 
When laying flat, shoulder seam to shoulder seam measures 16.5 inches. 
The measurement from the back/middle collar seam to the bottom of the shirt is 26.5 inches.  
This shirt is in nice vintage condition and comes from a smoke-free home.  Thanks for looking!