Ideally, you would cook the artichoke and then weigh the meat after you get it off the leaf. Unfortunately, that’s pretty tricky.
Other members of the Bright Line Eating community have gone through the effort to weigh the artichoke (whole), eat the meat, and then weigh it afterwards. They’ve found an 8 oz. artichoke results in 3 oz. of actual artichoke meat, so you can prepare 2 medium artichokes to count as a 6 oz. vegetable serving.