The Oxy dosing is sort of questionable, I've never been able really nail it down. But the consensus seems to be every 3 days.
Amoxicillin dosing for chickens is 57mg per pound of weight given orally twice a day for 7-10 days.
Personally, I would give the Amoxicillin - easier to dose and I think it would be more effective.
You'll need to try to weigh her if possible.
For dosing. Crush your tablet up finely or if you have capsules empty it out. Mix the powder with something "known" like 1 teaspoon coconut oil. Then divide that into equal parts.
For example - if she's 1 pound, then you would divide that into 8 parts which will get you close (good enough) to your 57mg (there's a bit of wiggle room in dosing Amoxicillin). Roll each part into a little ball, then put in the fridge to harden the oil. 1 ball is going to be her "pill" or dose that you would give 2x a day.
Does that make sense?
Poor girl. She need hydration badly. Will she drink on her own if you hold a cup up to her beak? You can also try crop/tube feeding to see if that helps. Fluids first of course. Some use Kaytee baby bird formula to tube feed.
https://www.backyardchickens.com/ar...dications-to-all-poultry-and-waterfowl.73335/
https://www.backyardchickens.com/articles/crop-feeding.75454/