Here's the thread:
https://forum.atlas-games.com/t/healing-wounds-when-you-have-increased-size/4317/1
The consensus was that the wound you took at the increased size would not worsen or better when you changed size. So a wound that would have been dead at size 0 is Incapacitating at size +3, if you used a spell to make your size +3 before the wound landed, when you reverted in size to 0 again the wound would still only be Incapacitating.