Halter Sizes? (20 characters)?

Hi, I want to get a leather halter for my section D welsh cob. She is about 15 hh. maybe a hair less. The sizes are o/s, c/s, and f/s, the only thing is, is that I am not sure what size to get. I have a Nylon halter by Weaver in the size "Average Horse" if that helps. Any suggestions on what size to get? Thanks!