Must a PC (or other creature) be Huge in size to deal the 3d12 damage dice upon a hit with a Huge-sized greataxe as per the oversized-weapon rules?

Question

Must a PC (or other creature) be Huge in size to deal the 3d12 damage dice upon a hit with a Huge-sized greataxe as per the oversized-weapon rules?

I believe the oversized-weapon rules are somewhat ambiguous as to whether the extra damage dice are a property of the size of the weapon (by itself) or result only when a big weapon is wielded by a similarly big creature. The text at issue is in the DMG under the chapter entitled "Creating a Monster"->subchapter "Creating a Monster Stat Block"->part "Step 11. Damage"->subpart "Base the Damage on the Weapon":

Big monsters typically wield oversized weapons that deal extra dice of damage on a hit.

This sentence suggests that the extra damage is a property of the weapon itself, independent of the size of the monster wielding it.

Double the weapon dice if the creature is Large, triple the weapon dice if it's Huge, and quadruple the weapon dice if it's Gargantuan. For example, a Huge giant wielding an appropriately sized greataxe deals 3d12 slashing damage (plus its Strength bonus), instead of the normal 1d12.

Here, the first sentence implies that any creature can use the oversized weapon, but it also seemingly links the number of damage dice to the size of the wielding creature instead of to the weapon itself. The example also specifies that both the greataxe and the giant are Huge sized. Still, especially given the next paragraph, by referring to the creature's size, the first sentence might best be read as referring to the weapon via the "appropriately sized" creature--thereby essentially categorizing the weapon itself as Large, Huge, or Gargantuan.

A creature has disadvantage on attack rolls with a weapon that is sized for a larger attacker.

This confirms that other creatures may wield the oversized weapon, but it does not clarify whether a successful attack would result in the additional weapon damage dice.

It looks like similar topics have been discussed before, but I didn't see anything directly answering the question.

Answer 1

This is up to the DM.

To directly address your specific question, "Must a PC (or other creature) be Huge in size to deal the 3d12 damage dice upon a hit with a Huge-sized greataxe as per the oversized-weapon rules?", it is up to the DM to determine if you can wield a weapon at all, and if so, how much damage it does. That's the short version, keep reading for the details...

There is no rule for oversized weapons.

At least, not in the way chapter 9 of the Player's Handbook presents rules for the players. The section that you quote is not proper rules text. Rather, this section that refers to oversized weapons is part of the instructions for creating a custom monster:

Creating a Monster

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The Monster Manual contains hundreds of ready-to-play monsters, but it doesn’t include every monster that you can imagine. Part of the D&D experience is the simple joy of creating new monsters and customizing existing ones, if for no other reason than to surprise and delight your players with something they’ve never faced before.

[...]

Creating a Monster Stat Block (Steps 11-15)

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Step 11. Damage

A monster’s damage output—the amount of damage it deals every round—has a direct bearing on its challenge rating, and vice versa. You can determine a monster’s damage output in one of two ways.

[...]

Base the Damage on the Weapon. Alternatively, you can use a die expression to represent the damage that a monster deals with each of its attacks based on whatever weapon it is using.

[...]

Big monsters typically wield oversized weapons that deal extra dice of damage on a hit. Double the weapon dice if the creature is Large, triple the weapon dice if it’s Huge, and quadruple the weapon dice if it’s Gargantuan. For example, a Huge giant wielding an appropriately sized greataxe deals 3d12 slashing damage (plus its Strength bonus), instead of the normal 1d12.

Again, this is not rules text. These are instructions for creating your own stat blocks, and gives some insight into how the authors typically selected damage dice when designing monsters. But this is not a rule to be invoked at the table in the same way the player-facing rules of the PHB are, and the authors didn't even follow it all of the time when designing monster stat blocks. For example, the fiend Baphomet, size Huge, carries a glaive named Heartcleaver:

Baphomet wields a great glaive called Heartcleaver.

Heartcleaver's attack is described:

Heartcleaver. Melee Weapon Attack: +17 to hit, reach 15 ft., one target. Hit: 21 (2d10 + 10) force damage.

Since Baphomet is a Huge monster, we would typically expect the weapon attacks to deal 3dX, but Baphomet's do not. So this bit about oversized weapons is not a rule. It's a design principle that was usually followed, but not always.

So a huge creature does not always deal 3d12 while wielding a huge greataxe. The damage they deal will be determined by the specific effects at play and the description of the weapon or attack used.

That said, the DM can implement the guidance here as rules for the players, and this section does provide a rule to follow if you choose to do so:

A creature has disadvantage on attack rolls with a weapon that is sized for a larger attacker.

This rule is provided because of the inevitable player question that arises when the DM implements monsters with oversized weapons: can I hit stuff with that? The idea here, again, is that this information is not presented as rules text, it is presented as a DM-facing DIY guide for making monsters, which leaves it in the hands of the DM to use what they want from this guidance at the table, and the player's cannot bring to bear an expectation that it be followed, at least, not in the same way they can expect the basic combat rules to be followed.

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This answer was largely adapted from my answer to this question: How do Enlarge/Reduce spell interact with the Oversized Weapons rule when you Enlarge a Medium size creature with a medium weapon to Large size?

Answer 2

The quoted text does not apply to PCs

There are no rules for oversized weapons in 5e and it’s not a sort of weapon the PCs can wield. The text that you are quoting is not a game rule but part of guidance for the DM on creating and balancing custom monsters and it is not even always followed when it comes to official monsters’ stat blocks from the Monster Manual so don’t expect to be able to invoke it at the table.

PCs’ weapons deal the damage that is stated in their descriptions which can be found in Player’s Handbook, as well as here: https://www.dndbeyond.com/sources/basic-rules/equipment. These are independent of the character’s size. If an effect that changes PC’s size also affects the amount of damage that they deal, it will state so explicitly, for example:

• Enlarge/Reduce:

(…) The target’s weapons also grow to match its new size. While these weapons are enlarged, the target’s attacks with them deal 1d4 extra damage.

(…) The target’s weapons also shrink to match its new size. While these weapons are reduced, the target’s attacks with them deal 1d4 less damage.

• Potion of Giant Size:

(…) Everything you are carrying and wearing also increases in size for the duration. When rolling damage for weapons enlarged in this manner, roll three times the normal number of dice; for example, an enlarged longsword would deal 3d8 slashing damage (instead of 1d8)(…)

Incidentally, Potion of Giant Size does increase your size to Huge and makes your weapons grow with you and, as a result, deal triple damage and this is the closest reference that I could find for your question. It could probably be inferred from the wording that the increased damage is the result of the weapon itself growing in size. However, it could also be argued that logically the creature’s size would have to match the weapon for it to be able to use it effectively (or at all). The item’s description doesn’t state what happens to a weapon enlarged in such a way when passed to a normal-sized PC so it would be down to the DM to decide.