Do "Giant's Stance" and "Improved Natural Attack" stack as a Medium character?

Question

Giant's Stance states the following:

While you are in this stance, you deal damage as if you were one size larger than normal, to a maximum of Large. This benefit improves your weapon and unarmed strike damage. It does not confer any of the other benefits or drawbacks of a change in size, such as a modifier to ability scores or AC, or an improved reach.

while Improved Natural Attack states the following:

The damage for one of the creature's natural attack forms increases by one step, as if the creature's size had increased by one category: 1d2, 1d3, 1d4, 1d6, 1d8, 2d6, 3d6, 4d6, 6d6, 8d6, 12d6. A weapon or attack that deals 1d10 points of damage increases as follows: 1d10, 2d8, 3d8, 4d8, 6d8, 8d8, 12d8.

Giant's Stance does have an upper limit (Large), therefore, as a Medium creature, with Improved Natural Attack making the character already Large, Giant's Stance would have no effect. However, my thinking is that Giant's Stance would make the character Large and after that, Improved Natural Attack would be counted as one step higher; that is, Huge.

Is this correct or do both affect the base size (that is, both convert from Medium to Large) and are therefore redundant?

Answer 1

Improved Natural Attack doesn’t do anything to your size, it just increases your damage die size one step—which is an unusual operation in D&D 3.5e, so the authors “helpfully” make an allusion to a more-common operation, size increase, to give the reader an example of the kind of thing that is meant. It does nothing to your size, and therefore has no interaction with giant’s stance, or any other (effective-)size-increasing effect—both things just work as they say they do without complications.

In effect, this is “as if” you were Huge, but really, it’s much more “as if” you were Large with a slightly-better weapon (or, in this case, had a slightly-better unarmed strike, as you might if you were a [higher-level] monk or pugilist). It’s not even clear that Improved Natural Attack would be stopped even if you were already (effectively) Colossal—things cannot be larger than Colossal,¹ but Improved Natural Attack really doesn’t change your size, so it should still increase your die size by one (though some will argue that the “as if” here carries over the implicit limit at Colossal—I just don’t find this argument very compelling, because again, you could increase the die as if you were wielding a better weapon).

Be aware though, that although this works, die sizes increase in expected damage very slowly—for most sizes, it’s an average of +1 damage. If you stack quite a lot of die-size increases, it can be more significant than that, but by that point you have invested a lot in it—and that limit at Colossal winds up being a problem, limiting how much upside you can really get from it.

1. _{Except dragons, because of course. Epic Level Handbook’s “Colossal+,” “Colossal++,” and so on categories are weird and usually not worth considering.}

Answer 2

These will stack because Improved Natural Attack uses the language: "as if the creature's size had increased", but doesn't actually improve any sizes.

That means you're still Medium, your unarmed strike is still Medium, it just deals more damage than it normally would on a Medium creature.

Using Giant's Stance, you'll still be able to up it by another size category.