Object scaling with keyframe:
Create BP actor, name as BP_Cube, change the mesh to cube
In construction script (to adjust size by using variable):
Change category to Control (optional), so in Details, Size → the display show as control too
Make Size variable float and visible
To set keyframe animation
In event graph (rename static mesh to cube, Scaleup renamed by node Add Timeline) :
Open ScaleUp node to create keyframe (here 2 keyframe, scale up size from 0 to 5 form 0th sec to 5th sec)
Track name is renamed by R click on it