Script that randomly generates a platform in the scene with a random size and position.

using UnityEngine;

public class PlatformGenerator : MonoBehaviour
{
    public GameObject platformPrefab;
    public float platformWidth = 5f;
    public float platformHeight = 1f;
    public float platformDepth = 5f;
    public float platformSpawnRadius = 10f;
    
    private void Start()
    {
        Vector3 platformPosition = Random.onUnitSphere * platformSpawnRadius;
        platformPosition.y = 0f;
        platformPosition += transform.position;
        
        Vector3 platformScale = new Vector3(
            Random.Range(0.5f, 2f) * platformWidth,
            platformHeight,
            Random.Range(0.5f, 2f) * platformDepth
        );
        
        GameObject platform = Instantiate(platformPrefab, platformPosition, Quaternion.identity);
        platform.transform.localScale = platformScale;
    }
}