Abyssal Warlock on Forest Gnome (because it's a small race made smaller)
-5% height so the effect is more apparent
spam spells until you surge
now you're changed back to normal height
The specific surge was "Abyssal Surge: Haste: Caster" which caused this to happen on PGCC. Other surges seem to do the same thing but because it takes a while to test, I haven't recreated this many times.