In this Unreal Engine 5.3 Niagara basics tutorial, we explore how to control and configure Initial Sprite Size in your particle systems. Learn how each sizing mode works—from uniform sizes to non-uniform randomized dimensions—to get total control over the look and feel of your VFX particles! 
https://dev.epicgames.com/community/learning/tutorials/7xmY6/unreal-engine-ue5-3-niagara-basic-initial-sprite-size-19