Hello,
Sadly we can't animate the body scale in the timeline. It's been something I've requested since I work on iClone 7 in september 2019 : Change the size of a character in the timeline - Feedback Tracker (reallusion.com)
I encourage you to vote the new issue I made for iClone 8 in june 2022 even if I'm loosing hope about that feature : Change the size of a character in the timeline - Feedback Tracker (reallusion.com)
For the moment you can use some tricks :
-change the size of the environnement/objects to give the illusion that's the character which is changing size if the camera is correcly positionned. (but can work only if there is one character in the scene.)
-The second tricks would be to make the animation of a character in iClone, then export the character with the animation, then import the character you just exported. It won't be anymore a CC3+ character, but at least you can make the changing scale effect in the timeline with the character's animation.
Good luck!