How much penicillin do you give your 1000 pound horse?

This will depend on the penicillin product you are using and the type of condition you are treating. Different products contain different concentrations of the drug and even different forumlations which require different dosages. There is often a dosing range and the veterinarian makes a recommendation on the dose to use in a given situation based upon the type of condition/infectious organism being treated. Consult your veterinarian to determine an appropriate dosage in a given situation or if penicillin is even the appropriate drug for use in that situation.