I've used 1/2cc for large adult fowl, 1/4cc for small or younger pullets.
If injecting it (it can be given orally...but injecting is faster reacting and easier), be sure to change the injection site each time you dose. It's thick stuff, it stings/burns (but birds don't seem to react when it's injected...they can seem sore afterwards or next day). Some people inject sub-q some people inject intramuscular... either will work, don't go too deep intramuscular. (like in the breast)
Inject once daily, for 5-7 days. I prefer to treat a solid 3 days after the last symptom is displayed. So, if no more symptoms on day 3, I dose 3 more days.
IMO, there is no need to inject anyone in the flock that is NOT showing active symptoms. Only treat those displaying symptoms.
If you have some that are not sneezing/coughing, etc... don't treat those. It really won't do any good at all to treat them. Basically you are treating the secondary infections-- because the CRD illness won't be 'cured' by any treatment. If there are some that are not sneezing, coughing, or otherwise displaying respiratory symptoms, there's just no need to treat and no benefit to treat them.
they have it all.
Just a word though, most respiratory illnesses in poultry are not curable. They have to run their course and then go away on their own, some of these meds will help keep secondary problems from coming up, but in the long run, it will generally take 2 weeks or so for them to get over it. Now the bad part, 75% of all respiratory illness are then always carried by the birds, so they will continue to spread it to others in your flock even if they show no signs of it.
This is why a lot of states now are requiring your flock to be tested clean for Mycoplasma diseases by the NPIP .