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

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.
• Apr 21, 2021 at 13:33
• The title does not reflect the body of the question: the latter refers to very specific spells/features, while the title seems to ask about interactions in a broad sense. Apr 21, 2021 at 14:40
• That's a good point. @Eddymage I had meant the two listed features as an example. An edit that offers other examples would be good. Apr 21, 2021 at 15:21
• Per my experience here, usually question should cover specific situations, because it is quite hard to state a completely general rule about a broad topic, as the one you pointed out. I edited the title, see if it suits you! Apr 21, 2021 at 16:00

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".

• This sounds like it's a case of "Both make you bigger, with one having a requirement, at the moment the feature is used, rather than sustaining that enlargement throughout the feature's duration." Which sounds like it's specific to these two features, as opposed to a feature that might say "throughout the duration, you are one size larger than you'd be otherwise" which is cumbersome wording, but would result in a perpetual effect rather than an instantiated effect. More research required... Apr 21, 2021 at 15:11
• @PaydenK.Pringle Yeah, that's why as I commented under your question is usually better to ask a specific question. Apr 21, 2021 at 16:01

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.

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).

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

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.

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.

• Regarding "changing your size more than once", this implies that an Enlarged creature that has the spell suppressed by an Antimagic Field would then not become Enlarged upon exiting the Field. Though that's unrelated to this specific question, I thought it interesting to point out. Apr 21, 2021 at 13:57
• @PaydenK.Pringle Seems like a good comment to make under that answer Apr 21, 2021 at 14:04