This Aussie company makes a similar product and their sizing charts are easy to follow by breed. Maybe this might help?
http://www.cockcollars.com.au/size-charts
Hi, I already have two buff orpingtons who have just started laying in recent weeks. I've been fighting chicken math, but we'd love to have a BCM.
Will you have some chicks available in the spring? We can come out from NYC to pick up. Preferably around the last week in April.