currently debating at the table. Can a small creature knock a gargantuan creature prone? Yes, unless ability specifically says otherwise
— Mike Mearls (@mikemearls) October 29, 2017
currently debating at the table. Can a small creature knock a gargantuan creature prone? Yes, unless ability specifically says otherwise
— Mike Mearls (@mikemearls) October 29, 2017
As Mearls points out, it depends on HOW you’re trying to knock the creature prone.
The simplest and most direct method – a Shove attack – doesn’t work, because it specifically says you must be no more than one size category smaller.
Spells, of course, work exactly how spells say they do. If you have a spell that can knock things prone, and it makes no mention of size, it affects all sizes.
A number of creatures and monsters possess abilities that allow them to knock targets prone. Like spells, if they don’t mention size, it works on all sizes.
Aside from that, there are class abilities like the Battle Master’s “Trip Attack” maneuver – however, that specifically states the target can’t be bigger than Large.
In general, a Small creature is going to need some sort of special, probably magical assistance to knock a Huge or Gargantuan creature prone.
I’m currently playing a halfling Way of the Open Hand monk in the a Storm King’s Thunder. Flurry of Blows allows you to force a crate to make a Dex save or be knocked prone. To date, I have knocked down over half a dozen giants of various races. My character thinks of it as one of his signature moves.