I don't know my way around the XML or anything, but all the information you need is in-game. You can tell it's a small hit because it's always located near the middle of the list of diplomatic modifiers/at the top of the negative modifiers with a civ. If my understanding of the list is correct, that means it's not only small, but it's actually the smallest. It's also written in the darker red.
You can tell that it's temporary by scrolling over "you will pay for this." The tooltip says outright "you will anger [civ] temporarily." And I can confirm that it's accurate, as I always choose this option and the penalty always goes away. Relatively quickly too.