Is a 5 mm kidney stone considered big or small?

Not small, not big: Stone size is important in trying to predict which stones will pass on their own. Stones 4 mm and smaller have an 80% chance of passing on their own. As the stone gets larger, the chance it will pass all by itself goes down.

Borderline: Small enough to possibly pass on its own (usual case), too large if it gets stuck ; requires medical intervention. Good luck!.