Considering how common magic is in my own lore, size changing is fairly common to have, although rarely used unless a certain large character wanted to shrink to normal size to fit in say a house. Mostly because being big is commonly a disadvantage unless you are strong enough.
Only character who actively uses it in lore are the beings who created the universe.
Otherwise in rps, obviously my sona does coz he’s the common one I use and he is a villain for obvious reasons of being a demon, so he changes size of himself or others.
Ah, well I also have fantasy characters for it as well. However, everyone cares for the Bradanska family on DA (Grief, Scylla, Grau, and Ophelia). So they're my mascots. It's the opposite of my story, but then characters like Helga want to be normal size, for my others it's almost always meaning they're super strong and can shrug off bullets like nothing.
I have "aliens fucked our ancestors and now we're all hybrids that can manipulate matter with our minds", the best part is that this isn't the weirdest part of the Bradanska family, on the other side of the family are psychic witches.