Normal vectors are usually versors, so the magnitude is one unit.
What is the magnitude of a normal vector of a plane?
The magnitude of a normal vector to a plane is essentially the length of that vector. In the context of a plane in three-dimensional space, the normal vector is a vector that is perpendicular (at a right angle) to the plane.
By definition, the normal vector is only used to define a direction, and it is a versor, which means that the magnitude must be 1 unit.