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?