What are you wanting to treat for? If you want to treat for giardia, you can give that amount twice a day.
The math is:
5.25 pounds / 2.2 x 25 = 60 mg
60 / 200 = 0.3 grams twice a day
At that weight I would give her 120 mg, which is 0.6 grams of powder.
It should not affect her appetite.
If she's not eating and drinking like she usually does, you need to start weighing her daily and you should think about tubing fluids and food. Would you like to learn how to tube feed?
How much does your hen weigh? That one amount of powder there has 856 mg of metronidazole.
Let us know how much the hen weighs and I will tell you how much powder to use.
If you have a small scale, weigh 1 teaspoon of powder and let me know what it weighs. One teaspoon of most powders weigh ~3 grams.
Your powder says it's 20% metronidazole, which means there are 200 mg in one gram. The bag also says the one teaspoon = 5 grams, but I'd bet money that it's closer...