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:
- A Hill Dwarf Rune Knight activates Giant's Might, causing his size to become Large.
- His ally casts Enlarge/Reduce on him, using the Enlarge Option, causing his size to become Huge.
As opposed to inverting the order:
- Enlarge/Reduce causing his size to become Large.
- 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:
- Giant's Might, causing his size to become Large.
- Enlarge/Reduce, causing his size to not change, because Giant's Might forces it to be Large.
Best Answer
The order does count.
As you reported, the description of Giant's Might says (emphasis mine):
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):
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".