This post will tell you how to change the size of the player in Unreal Engine 4.25. I used the third person template project.
To start, open your player (third person character for me). You want to make the code like the following
On event (f on keyboard) – set world scale 3d. Set the target to (capsule component). Put in the scale you want your player to be. In my example, my player will grow and shrink on different keys, and when you release the keys the player will become normal sized again.