How do Giant's Might feature and Enlarge/Reduce spell interact?

Question

Rune Knight's Giant's Might states:

If you are smaller than Large, you become Large, along with anything you are wearing.

(Note: I am linking to the UA because it is freely available and has the same wording for the feature related to this question.)

The spell Enlarge/Reduce states:

Enlarge. The target's size doubles in all dimensions, and its weight is multiplied by eight. This growth increases its size by one category - from Medium to Large, for example.

Is it simply a matter of order of operations? Interpretation A:

1. A Hill Dwarf Rune Knight activates Giant's Might, causing his size to become Large.
2. His ally casts Enlarge/Reduce on him, using the Enlarge Option, causing his size to become Huge.

As opposed to inverting the order:

1. Enlarge/Reduce causing his size to become Large.
2. Giant's Might, causing his size to not change.

Or does one feature override the other resulting in Interpretation B, due to Specific Beats General:

1. Giant's Might, causing his size to become Large.
2. Enlarge/Reduce, causing his size to not change, because Giant's Might forces it to be Large.

Answer 1

The order does count.

As you reported, the description of Giant's Might says (emphasis mine):

If you are smaller than Large, you become Large, along with anything you are wearing.

If someone has already cast on you Enlarge/Reduce making you Large, if you try to use Giant's Might then the size requirement is not met, hence you remain simply Large.

There is no Specific beats general here.

On the other hand, Enlarge/Reduce does not have any limitation on the starting size (emphasis mine):

Enlarge. The target's size doubles in all dimensions, and its weight is multiplied by eight. This growth increases its size by one category -- from Medium to Large, for example.

If someone casts this spell on a creature that has activated Giant's Might, being hence Large, due to the fact that there are no actual limitations on the starting size that creature becomes Huge. Moreover, the description of Giant's Might does not present any limitation about a further size increasing, such as something similar to "The creature's size can not be greater than Large while this feature is active".

Answer 2

Giant's Might cannot override Enlarge if you no longer meet the condition for Giant's Might.

Giant's Might states:

If you are smaller than Large, you become Large

If you use Giant's Might to become Large, then cast enlarge/reduce to become Huge, Giant's might does not force you back to Large because it says you become Large only if you are smaller than Large, but you are Huge.

If you cast enlarge/reduce first to be come Large, then Giant's Might does not change your size, as you are not smaller than Large.

Answer 3

Technically, order matters

Other answer showcase this well enough but say enlarge/reduce was applied first, then you are already Large and becoming Large again (using Giant's Might) simply does nothing. Whereas, as already demonstrated in your question and its other answers, doing things the other way around means you would first become Large and then be enlarged into being Huge (and nothing in the text of Giant's Might prevents you from becoming Huge at a later point in time).

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The enlarge/reduce spell might change your size more than once

We have a related question about this sort of scenario:

• Does an Enlarge/Reduce spell on a druid affect the beast form if the druid Wild-Shapes?

From the most-upvoted answer, you can see arguments made that enlarge/reduce only takes effect once and from the second-most-upvoted answer, you can see arguments made that it is a mess and a GM is going to have to decide whether enlarge/reduce can effectively apply more than once.

That said, we definitely want to avoid having the spell apply indefinitely, so a GM could determine some sort of set of circumstances where the spell does or does not re-apply, or they could just go with their gut and the ideas of fun and balance. So a GM has room (even within the rules) to make the order not actually matter.

In fact, perhaps the spell even should constantly apply in some sort of way since one would certainly hope it re-applies after exiting an antimagic field.

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You don't have to follow these rules, at all

Having something like order of operations matter with features like these just because it's what the rules literally imply does not help improve the game (and even then, again, there's room to argue enlarge/reduce re-activates). It makes your players have to perform actions in a seemingly arbitrary order because of something the players know. Their characters, however, do not know the wording of these features and have no reason to believe these would not work in either order.

No harm is caused by allowing these features to work regardless of their order of application when one of those orders already works perfectly well, so houserule away and let the players have fun.

Answer 4

Your question missed an important quote from the original rule, which is where your confusion is coming from.

This is the important bit, quoted from your link. Emphasis mine.

At 3rd level, you have learned how to imbue yourself with the might of giants. As a bonus action, you magically gain the following benefits, which last for 1 minute:

The bolded line makes it clear. While the effect is active, the following is true of your character;

If you are smaller than Large, you become Large, along with anything you are wearing. If you lack the room to become Large, your size doesn't change.

At any time your character is smaller than large, Giant's Might makes you large. If someone cast Reduce on your character, and the minute still wasn't up, then the ongoing effect of Giant's Might would see that you're smaller than large and once more make you large. The inverse, however, is also true. If you activate an effect which makes it so you are no longer smaller than large, then Giant's Might stops making your base size large. Thus, if Enlarge is cast on someone with an ongoing Giant's Might effect active on them, then said Giant's Might effect would see the character is no longer smaller than large, and stop making them large. With no effect actively making them large to begin with, the character would revert to their base size. Enlarge would still be active, which would increase their size category by one. If their adjusted size category under Enlarge is large, then they remain large and the ongoing effect of Giant's Might continues to see that they are not smaller than large.

This is also in line with what happens when other set-value modifiers and value-adjustment modifiers are both put on a character at the same time. Such as with an Amulet of Health (which makes the character's Constitution score 19) and a Belt of Dwarven Kind (which, among other things, increases the character's Constitution score by 2) not giving the character a combined total Constitution score of 21, regardless of which item is attuned to first.