Horn worms are soft, but quite large, so I think as long as the beardie is at least 15-16" long, the hornworms are fine.
You should be aware of the fact that hornworms grow extremely quickly. I bought a few once at an expo and they tripled in size in a week. Also, they poop *a lot* and it does not smell good. And one of them bit me once. :?
I don't think there's a risk of impaction since they are soft worms, but since they get so big, so quickly, I think feeding them to a smaller dragon isn't the best idea. And I don't think the worms will bite your dragon, the one only bit me because I was trying to pry it off its little screen/perch thing and it freaked out (this was a full-grown worm, I never had that happen until they were huge). Your dragon will probably snatch it up and kill it before it could even start to react.