How to make an area instead of a point?

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

public class FlySpawner : MonoBehaviour {

    [SerializeField]
    private GameObject flyPrefab;

    [SerializeField]
    private GameObject player;

    [SerializeField]
    private int totalFlyMinimun = 12;

    [SerializeField]
    private float spawnArea = 25f;

    private Vector3 nonSpawnArea;

    public static int totalFlies;

    // Use this for initialization
    void Start () {
        totalFlies = 0;
    }

    // Update is called once per frame
    void Update () {

        // While the total number of flies is less than the minimum...
        while (totalFlies < totalFlyMinimun) {

            // ...then increment the total number of flies
            totalFlies++;

            // ...create the area in which the flies can not spawn in
            nonSpawnArea = new Vector3 (player.transform.position.x + 3f, 2f, player.transform.position.z + 3f);

            // ...create a random position for a fly...
            float positionX = Random.Range(-spawnArea, spawnArea);
            float positionZ = Random.Range(-spawnArea, spawnArea);

            Vector3 flyPosition = new Vector3 (positionX, 2f, positionZ);

            // ...check that the fly isn't spawning on top of the player
            if (flyPosition != nonSpawnArea) {

                // ...and create a new fly
                Instantiate(flyPrefab, flyPosition, Quaternion.identity);
            }
        }

    }
}