I'm writing a campaign about which features a Champion of Chaos as the BBEG and I'm trying to find a sourcebook that can quantify (or at least give me an idea) how strong they are or should be. It's possible I'm using the wrong term for a human that serves and has been chosen by a chaos god, but that's basically what I'm looking for.

See Black Crusade. Even a starting human character is an elite when compared to your typical Dark Heresy character. They also are free-form advancement, so give your champion whatever you want, as that line has all sorts of rules for "gifts" from the gods, chaos sorcery, tainted weapons and armor, and general warp shenanigans.

Various npcs are statted out as well, if you want an idea of power scale.

Also, technically, you are using the proper term, as a powerful worshipper of chaos is a champion. When most 40K people hear champion though, they generally think of a Chaos Space Marine, as they have minis with that name.

And the obvious, whichever God they are aligned with, give them bonuses based upon their affiliation.

Mutations, gifts of the Dark Gods (out of Enemies Beyond), Daemon Weapons, high-level gear, all are good options.

If you're lucky enough to own. Or find. A copy of Slaves to Darkness or Lost and the Damned then get them, someone the best things GW ever published for this topic

Tome of Corruption was a good one too.