Equiture Browband Size Guide: 

Browbands measurements are taken when browband is flat on the table and straight across from loop to loop (we don't follow the curve of the browband if the browband is a curve). 

Pony: 14" 
Cob: 15" 
Full: 16" 
XFull: 17" 
XXFull: 18" 

Sizes include loops which are 1.5" per loop which accommodates most comfort

Tommy Hilfiger Equestrian Size guide: 

Eskadron Size Guides: 

Pikeur Women's size Guide: 

Equestrian Stockholm Size Guides:

PS of Sweden size guides: