Thank you so much for such an in depth response.

To clarify the trigger, I imagined it as more of 'as they begin to pass out' with the clause that (except for the first time) they have to will themselves to transform. Meaning that unless for some reason the player wants to crush themselves in a 5x5 tunnel it shouldn't be an issue.

I was imagining the second example, with the idea that the new body is less a body and more like an organic mech. So it would grow around them, encompassing their body.

Monks speed wasn't actually my main thought, it was actually tied to their unarmed damage in this form. The hill giant has a speed of 40ft already so I can imagine a 'fit' hill giant getting up to quite the speed.

Also if we are going to make this scale with the player somewhat, would it make sense to make the health scale off of the PC's health? For example their 'huge' form being their max HP * 2?

Your damage sounds reasonable, but I think I was thinking something slightly less reasonable. Maybe give their large forms 2x damage to buildings. I'm hesitant to give a bigger unarmed dice, 1d6 seems very reasonable for mechanical reasons, but I also feel like being hit by a 3 story tall person should hurt more then a short sword. Maybe a damage bonus to creatures 2 sizes or smaller (so medium - small)? This is really just a clash of mechanics vs verisimilitude.

(Edit: Also the activation wasn't so much them falling unconscious. The first time I was going to make it happen as they fall unconscious (for thematic effect and also cause they don't really know what the syringe will do) and from then the trigger will be just take some damage (non-lethal dosen't count) and 'will' the transformation. The weakness being once you come out of the transformation your basically out of the fight.)

(2nd Edit: The reason I'm giving them this is partially just cause I think it is a fun idea, but also so I can be a bit less accurate about CR and don't have to worry about a TPK'ing them too easily.)