These t-shirts are a relaxed fit in standard shirts sizes and are a good style for both men and women.  How you wear your t-shirt - fitted vs relaxed - would dictate if you order up a size or down a size (which explains the size overlap in my chart).  As a general rule, I created the below size chart to correspond with women's dress sizes.  

Small size 2-6

Medium size 6-10

Large size 10-12

XL size 14+