I exracted some gelatin from bones and I want to make “Meat-Mallows” but the marshmallow recipes all call for powdered gelatin, so how much liquid gelatin is equivalent to a pack of Jell-O? Or how do I turn my homemade gelatin into powder?
This is not an answerable question.
how much liquid gelatin is equivalent to a pack of Jell-O
This depends on the concentration, and you don't know it. It's probably not even worth trying to measure it, as I suspect that it will differ quite a bit between batches.
how do I turn my homemade gelatin into powder
This is probably doable, but not easy. You'd have to explore some kind of homemade vacuum dryer, as a dish of bone extract in a normal dehydrator would be roughly equivalent to a lab incubator for bacteria. But the more important part is that this won't help you further, because removing some water from your self-made gelatin won't let you standardize its binding strength in any way.
If I were you, I'd use the homemade extract for less sensitive applications, maybe some kind of aspic, and follow the meat-mallow recipe without any substitutions.