If a character starts using Giants Might in an enclosed area, then enters a more open area, will their size increase?

Question

The RAW say:

Giant Might 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:

• 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.
• You have advantage on Strength checks and Strength saving throws. Once on each of your turns, one of your attacks with a weapon or an unarmed strike can deal an extra 1d6 damage to a target on a hit.
• You can use this feature a number of times equal to your proficiency bonus, and you regain all expended uses of it when you finish a long rest.

A tangential post, asked about using Giants Might in a 5' wide space but also using the 'squeeze' option for large creatures in small spaces. The answers to the question of favored DM's discretion, but gave interesting background on the issues of enlarging characters an objects.

Also (somewhat?) related is this post, asking about voluntarily ending Giants Might. I agree with the answer that adheres to the RAW: since there's no mention of being able to do so, you can't--the condition lasts for a minute because it says it lasts for a minute, and doesn't say anything about ending it voluntarily.

My guess is this will similarly be viewed as a matter for 'DM's discretion'. If so, what could DMs base their decisions on?

Are there similar spells or abilities that speak to the issue?

Answer 1

Using only info and guidance given in the description of Giants Might, one reading would say movement to a more open space does NOT allow a delayed increase in size because the rules don't say that it does. According to this reading, if it couldn't happen initially, that's it; the size change is a part of the initial moment of enacting the ability and is set at that time. While that view is a simpler reading (compared to what I offer below) it IS still an interpretation.

A second view focuses on the 'If' in the key sentence,

"If you lack the room to become Large, your size doesn't change."

It says the 'If' implies that change in size is conditional based on the immediate environment (i.e. NOT set and fixed a the initial moment of activating the ability). While both the first view and this second one are interpretations of the RAW, the second view raises more complexities and seems overall, the weaker of the two.

By the logic of 'change in size is conditional based on the environment', movement from an open area to a more limited/smaller area could/would cause a shift DOWN to smaller size. That could lead to shifts up and down, which is not at all described by the spell and, frankly, would be somewhat comical: imagine shifting up and down and up and down as you run through hallways and rooms and intersections of hallways (cue sound of a a balloon squeak as you go through a doorway). This stretches to the breaking point the logic behind this second view.*

Are there other readings to consider? What am I missing?

*one could argue for one-way shift along the lines of a cork-from-the-bottle analogy: your size can go up as space allows, but can't reduce down (a cork pulled from a bottle, is hard to put back). But while that avoids the comical shifting-up-down-up scenario, there's nothing in the RAW to point towards a one-way shift.