Accessing Inherited variable before it instanciation

Hello all ! Sorry if my english is bad, i'm not a native :)

I have a class Enemy.cs, then each type of enemy got his own script which derives from Enemy.cs, so I can override functions like Shot(), Move(), etc. I got a boolean "isBig", because for spawning purpose, some enemies have twice the size of others. With parameters in a function that is not shown here to simplify, I can spawn different enemies in the same time but I can't have more than 30 enemies that pop in the same time, so I need to calculate before instantiate them if my parameters are right. You have more explanations on the code annotations.

So if my conception is good, my question is : How can I ask the isBig variable value of the derived class of Enemy.cs before even instantiate them ? Is there a way to store data that came from a parent class and is override before the Game is started ? I don't know if I am clear so don't hesitate to ask question.

Here are my scripts : The parent class Enemy.cs, where i wrote the base mechanics for all enemies and avoid duplicate code :

    public class Enemy : MonoBehaviour
     {
         protected float _speed = 4.0f;
         protected float _fireRate = 2.0f;
         protected int _health = 1;
         protected int _damage = 5;
         protected int _score = 10;
         protected bool _isBig;
     
         protected Player _player;
     
         protected virtual void Start()
         {
             InitializeEnemy();
         }
     
         void InitializeEnemy()
         {
             _player = GameObject.Find("Player").GetComponent<Player>();
     
             if (_player == null)
             {
                 Debug.LogError("Player is NULL");
             }
         }
     
     public bool isBig()
         {
             return _isBig;
         }
 [...]
 }

SpawnManager where I want to have access to the "isBig" boolean of my child class :

 public class SpawnManager : MonoBehaviour
 {
     [SerializeField]
         private Enemy[] _enemiesPrefab;
         [SerializeField]
         private Transform[] _spawnPositions;
     
      void Start()
         {
     // Exemple where I have FALSE instead of TRUE, because my ennemy is not instantiated so it takes the ISBIG variable from the parent class, and I want the isBig from the enemy that I indicate. Of course I don't know in advance if it will be a big enemy or not (and so take the right script type directly) but this is to demonstrate that even if i know that, it will not work. 
     
             Big_Enemy bigEnemyScript = _enemiesPrefab[1].GetComponent<Big_Enemy>();
             Debug.Log(bigEnemyScript.isBig());
     
           //Here is the real purpose of this question, I pass in parameters that I want instantiate _enemiesPrefab[0] 25 times and _enemiesPrefab[1] 3 times. Since index 0 is an enemy which count for 1 size, and index 1 is an enemy that is 2 size ("a big enemy"), result will be 25x1 + 3x2 = 31. Instead of this, I got 28 because of 25x1 + 3x1 cause the isBig of the big enemy is take from his parent and not from him... 
             
      Debug.Log(CheckTotalEnemies(0, 25, 1, 3));
         }
     
     private bool CheckTotalEnemies(int firstEnemyType, int firstEnemyNumber, int secondEnemyType = -1, int secondEnemyNumber = 0, int thirdEnemyType = -1, int thirdEnemyNumber = 0)
         {
             int total = 0;
 
           // I multiply by two if it's a big enemy then I add to the total, or I directly add to the total if it's a 1 size enemy.
             total = _enemiesPrefab[firstEnemyType].isBig() ? total += firstEnemyNumber * 2 : total += firstEnemyNumber;
     
          // Add a second, or a third enemy type is optional
             if (secondEnemyType != -1)
             {
                 total = _enemiesPrefab[secondEnemyType].isBig() ? total += secondEnemyNumber * 2 : total += secondEnemyNumber;
     
                 if (thirdEnemyType != -1)
                     total = _enemiesPrefab[thirdEnemyType].isBig() ? total += thirdEnemyNumber * 2 : total += thirdEnemyNumber;
             }
            
          // I check if the total doesn't exceed the maximum enemy size which is 30
             return total <= 30 ? true : false;
         }
 }

And this the script of a big enemy which inherit from Enemy.cs :

 public class Big_Enemy : Enemy
 {
     protected override void Start()
     {
         base.Start();
         InitializeDefaultValue();
     }
 
     void InitializeDefaultValue()
     {
         _speed = 1.0f;
         _fireRate = 10f;
         _health = 1;
         _damage = 20;
         _score = 10;   
         _isBig = true;
     }
 }

The script of a simple enemy is the same without the IsBig line.

Thank you very much for any help ! :)