Congrats to your 100th post.
To aswer your question:
Looks like th main size is 16" (with a little give)
This means troopers with 17" would be snug, but comfy, and 18" would be tight.
if everything goes right, I'll even consider does a 18" version
For those with smaller necksizes, they'd simple cut a piece out and re-glue it with a bike tire repair kit.