I thought I saw this rule in an official D&D 5e rulebook, but now that I'm looking for it, I can't find anywhere. I'm pretty sure it was an optional/alternative rule, like in a little sidebar in the DMG or something.
The rule stated that if the size difference between two creatures was 3 or more size categories (so Medium to Gargantuan for instance), that the smaller creature would not be able to do any melee attack damage because their weapons would be too ineffective.
So my question is - is there such a rule in D&D 5e?
PS - I'm aware of the weapon damage increase/decrease rule for weapon sizes, but that's not what I'm asking about, I want to know specifically if there's a rule that states that a smaller creature can't harm a larger creature if there's the size difference is too great.