Horn worms are so 'squishy' that I don't think the conventional rule of thumb (i.e. no food items wider than the distance between eyes) applies. I don't necessarily have any guidance for you, but I can tell you this much... As soon as a Chameleon bites down on the body of a Horn worm, this 'squishiness' gets compressed quite a bit.
Just FYI, here's a picture of my Chameleon eating a nearly full-size Horn worm (i.e. ~2.5" in length) at 4-5 months of age.
At some point my Chameleon would simply refuse to eat the larger Horn worms... Almost like he knew when they were too big for him to eat. Perhaps someone with more expertise and/or requisite experience will chime in with some guidance for us newbies.