In the case of an suspected case of tetanus the dosage can range up to 3000units along with 1500units of tetanus toxoid given to effect . In other words
more of the antitoxin can be given to prevent or control the symptoms of the tetanus organism.
Size or age doesn't make the difference what you are treating is the effects of the tetanus.
it is unknown if this horse has been previously vaccinated with tetanus toxoid, will antitoxin still help until the tetanus toxoid is available?
Yes by all means give the antitoxin even or not if signs of tetanus is not prevalent then follow up with toxoid just to be on the safe side.