What is the hitbox on thresh's hook?

I don't know about Thresh, but Zilean bombs have a secondary AOE, that allows them to attach to units, displayed by the inner circle. When thrown, they detect in that radius, picking a unit to attach to, prioritizing enemy champs.

Idk but i know it's larger than the hitbox on warwick's ULT.

It's "Edge-to-Edge" hit box, meaning as long as the edge of the hook touches the edge of your character, it lands.

Meaning it's very busted for certain champions, such as Shyvana, who's hitbox is much larger than model.

Good Thresh player often abuse this.

All of you character

The bigger your soul is, the bigger the hitbox.

In all seriousness, it helps to see the abilities and characters' actual hitboxes, for example, no matter your character, the hitbox is at their feet, and you have to factor in that it's not a fully top-side camera but angled and all that blah blah.