I’m looking to buy a breastplate from Dy’on. It’s been while since I bought one and my current horse is petite cob/horse size (he’s an appendixy thing but we don’t have papers 
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I don’t have the breastplate you’re asking about specifically, but I do have a few Dy’on pieces (including a breastplate) and I feel like all of them are very true to size. I’ve been super happy with the quality!
I bought a Dy’on “working” bridle in cob size for a small Holsteiner who otherwise fits cob and it was too small, I had to add extra holes and there wasn’t much leather to do so. Not sure how that translates to your situation, but at least a word of caution 
I just bought this breastplate in full and haven’t actually attached it to a saddle but when I stuck it over my 16.1 TB’s head just to eyeball it looked like I was going to have to adjust it down to the smaller side of the available holes. There’s a fair amount of adjustability, I imagine if he’s in between sizes you could get away with either cob or full.
My mare is 15.3 but has shoulders (French Trotter lol) and the full fits her with room to adjust.
