I have made Gordon a few harness. I purchased one and just eyed it and then took typing paper and taped to sheets together and made a pattern. The one I bought him was x-small. I do know the girth area is around his chest.
I don't make any to sell, but I do know that some on here do......I'm sure they can help you.
Here are a few I made for Gordon:
__________________ Sherry Mommy to  Oliver Dupree  |