Horseloverz just had a mini neoprene cinch on clearance. The smallest cord cinches seem to be about 20". I have a 20" and a 22" for my donkey but he is 13hh.
https://www.horseloverz.com/product/cinches-and-accessories/148531-performers-1st-choice-miniature-horse-cinch.html

"It's really quite amazing what a horse will do for you, if he only understands what you want. 
And it it's also quite amazing what a horse will do to you if he doesn't." ~ Bill Dorrance