About 6". Bigger than that best to cut down ot double girdle and spray the stump or girdle.Yup, you beat me to it. I always used a hatchet on anything larger than my thumb, which was way over kill but I wanted to make sure the tree soaked it up. Trying basal bark treatments with the mix you've mentioned above is what I want to try this year. I'm curious how big of a tree a guy can do this on?