ReflectionProbe.size

Description

The size of the box area in which reflections will be applied to the objects. Measured in the probes's local space.

The box size will be scaled by the transform's scale.

using UnityEngine;

public class Example : MonoBehaviour
{
    void Start()
    {
        ReflectionProbe probe = GetComponent<ReflectionProbe>();
        // Make the area long
        probe.size = new Vector3(10, 1, 1);
    }
}