I'm making a combat which includes a Size 3 Ace, but i can't figure out how to actually change the Ace's size. I know it's possible, i've played a module with multiple resized mechs, so i'm just trying to figure out how to do it myself. I have a trigger which on combat start, sets the Ace's size to 3, but it's not working properly.

Huh, I’m not sure how a module might have done that; there’s currently no mechanism that the game supports to manually adjust mech sizes. They might have snuck a formorian frame on one somehow, but iirc the NPC kit picker in the current build doesn’t let you pick PC kits. Possibly they manually edited some json files?

I can put this on the request list, but until then you might have to open those modules in the editors and figure out how they did it.