If they are full grown and normal size they probably weigh at least 5 pounds. Would be best to weigh them. Can you post a video of them coughing?
Suggestion... you should make a new thread and have these posts about your chickens coughing moved to it. :)
The dose my vet had me use was 57 mg per pound twice a day. How much do they weigh? Who ever is telling you to give 16 mg once a day is wrong. The amoxicillin dose for chickens is much more than it is for cats and dogs, and it must be given at least twice a day.