（１）Random.valueを使う方法

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class RandomScale : MonoBehaviour
{
    private void OnTriggerEnter(Collider other)
    {
        other.transform.localScale = new Vector3(1, 1, 1) * Random.value * 3;
    }
}

（２）Random.Range()を使う方法

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class RandomScale : MonoBehaviour
{
    private void OnTriggerEnter(Collider other)
    {
        other.transform.localScale = new Vector3(1, 1, 1) * Random.Range(1.2f, 5f);
    }
}

（実行結果）

• 触れるたびにオブジェクトの大きさが変化すれば成功です。