We highly suggest using your regular shirt size when ordering.  

SM Sizing Fits Bra Band Size 32-34 

LXL Sizing Fits Bra Band Size 36-38

1x/2x Sizing Fits Bra Band Size 40-44 

2x/3x Sizing Fits Bra Band Size 42-46 

Ashley is a 40H while nursing and can easily wear the L/XL at a normal size Large Shirt size.  Sizing up to 1x/2x is great also.  You cannot go wrong in your normal size or sizing up one.