Agri penicillin seems to be 300,000 IU/ml. 300,000 IU = 297 mg. 1 pound = 0.45 kg Dosage for turkeys is 100 mg/kg.
297 mg/ml means 1/3 of a mL = 100 mg. Use half of that, because a pound is roughly half a kg, and you get that you need 1/6 of a mL.
If she's a little less than a pound, I would lowball to 0.1 mL of 300,000 IU penicillin, injected daily for four days.
Penicillin toxicity is a thing, so don't overdose. My Dad uses to think the more the better, (despite my insistence otherwise) until the vet corrected him.
You can do it intramuscularly (breast. Upper two-thirds.), but on such a small duck, I would rather do it subcutaneously. IM is fast-release and a bit painful. It'll hit the bacteria harder. SQ is slow release and will hit the bacteria longer.
If you have a different IU of penicillin, please mention it.
Second opinion,
@casportpony?