The
Dungeon adventure "Chadranther's Bane" in issue #18 is based on this exact scenario. I included a similar one in the 5e 2024 campaign that I'm currently working on. Note that changing one's size that much creates some weird impacts due to the height-mass discrepancy, but I sort of hand-wave that because it's "magic." Here are the game effects I used:
- Subjective scale is reduced to one eighth of normal; distances are calculated based on the characters' new size
- Falling damage is halved
- The short and long ranges of ranged weapons is halved (spell ranges are unaffected).
- Any magic that would change a character's size does not function
This adventure is in Fantasy Grounds, so it was easy to create a duplicate of the map and alter the scale. The foes were jermlaine and their pet rats; basically they became ogres and dire wolves respectively after the shift takes effect. Borrowing from "Chadranther's Bane," I had the effect radiate from a crystal in the jermlaine leader's room that the party has to destroy to reverse the effect.