How much damage do giants deal with unarmed strikes?

Question

A lot of monsters have attack descriptions that include using their appendages to hit characters and those appendages generally can't be disarmed. However, giants are unique in that they almost always attack with oversized weapons.

Per the DMG (pg 278):

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.

This question discusses ways to effectively disarm a giant and this one discusses ways to cheese Heat Metal to avoid the worst effects by just dropping the weapon.

Given that Unarmed Strikes for players deal a base of 1 damage barring something modifying it (i.e. being a Monk), if a character were to disarm a giant in some fashion, grab their weapon and choose to run away with it, how much damage would they be risking from an Opportunity Attack if they did so? Would it simply be:

• (1 * Size Modifier) + Strength Modifier

Is this correct or are there alternative rules which would apply that stipulate a higher base damage for Large and larger creatures' unarmed strikes?

Answer 1

There are no concrete rules, only some guidance.

As you quoted the DMG includes a section discussing the damage caused by oversized weapons. Two things to note is that this section is not some kind of hard rule, it is part of the chapter about creating your own monsters and provides only guidance on how to scale damage. The section also deals with weapon dice, whereas unarmed strikes generally do not include any dice at all (with exceptions, like monks), so it wouldn't even apply if it was a rule. In conclusion there are no clear rules on this except for keeping the damage as 1 + Str Mod per the general rule on unarmed strikes. Damage scaling by creature size would be up to the DM.

Your suggestion to scale the damage as (1 * Size Modifier) + Str Mod is very reasonable. But to offer an alternative with a bit of RAW support, a section in the adventure Storm King's Thunder speaks of unarmed hill giants and mentions:

If forced into melee combat, an unarmed hill giant can use its action to make two unarmed attacks. Replace the giant’s greatclub attack with the following attack option:

Unarmed Attack. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 12 (3d4 + 5) bludgeoning damage.

There is no general rule given but with Hill Giants being Huge creatures, which equates to a Size Modifier of 3, it provides an alternative scaling for creatures above a Medium Size: (Size Modifier)d4 + Str Mod.

Answer 2

This is not defined in the Core Rules

The Players Handbook lists normal unarmed Strikes as 1 damage in the weapon table on page 147.

The giant does not have natural weapons, it has manufactured weapon attacks, and if it loses the manufactured weapon due to player action, it will be reduced to unarmed attacks, just like a character (unless it has some rocks to throw...). This is covered in the Sage Advice Compendium:

Are natural weapons considered weapons? Things designated as weapons by the rules, including natural weapons, are indeed weapons. In contrast, unarmed strikes are not weapons. They are something you do with an unarmed part of your body.

The DMG advise on monster damage by type of attack is this (page 278):

If a monster has natural weapons, you decide how much damage it deals with those attacks, as well as the type of damage. See the Monster Manual for examples.
If a monster wields a manufactured weapon, it deals damage appropriate to the weapon. For example, a greataxe in the hands of a Medium monster deals ld12 slashing damage plus the monster's Strength modifier, as is normal for that weapon.

Unfortunately, an unarmed strike is neither a natural weapon, nor a manufactured weapon. The section you cite above is also about manufactured weapons. Because there is no rule that tells us what the damage of an unarmed strike for a larger than normal creature would be, the damage of such an unarmed attack is left undefined.

Also note that the rule for multiplying the damage die by size is not a hard rule, it is guidance only. For example Baphomet is a Huge demon, but his weapon attack, a great glaive called Hearthcleaver that deals force damage only deals 2d10 plus damage bonus, not 3d10.

It would seem reasonable to apply the same size multiplier to the unarmed strike, which deals a default of 1 point of damage, for 3 damage.

It also would be reasonable to compare this to other Huge monsters that have natural fist attacks that are similar to unarmed strikes, such as the Giant Ape, which deals 3d10 damage.

Or it would be reasaonable to assume that rowdy giants have a natural penchant for fistfights and grant them the equivalent of the Tavern Brawler feat, giving them in combination with the size modifer a base damage die of 3d4. This latter option is apprently what was chosen for the hill giants in Storm Kings Thunder (Kudos to @Pepijn who dug that up).

I think that this latter option is the best one, as it has precedent in published materials.