I remember watching this anime as a teenager, a few years ago. It was about a bunch of school kids who could do magic and had like a secret club in their school, and this girl was like an idol and her mother was a famous actress. She spent the whole episode not noticing she was a giant and minding her business before she returned to normal.
All I found are these two videos of the GTS episode, in case anyone remember the name watching the clips.
https://www.youtube.com/watch?v=jdIK1ggWWY8
https://www.youtube.com/watch?v=SXp-56vn9wY