Need help Adding Physics based recoil to my gun

In my game, I have 2 spheres that have a gun rotate around them. They are able to shoot and charge up shots with their guns. But what I want to do is add a recoil force based on the size of the bullet that they shoot. But I have been having SO MUCH trouble with getting the forces to work just right. If Anyone has any sort of input please let me know

Here's my code for the shooting and player movement: public class PlayerMovement1 : MonoBehaviour { //Global variable for all players public static float bulletSpeed = 3f; public float sizeIncreaseRate = 0.4f; public float playerSpeed = 10f; public float jumpSpeed = 50; public float recoil = -25;

 public GameObject pivot;
 public GameObject bulletEmitter;
 public GameObject bullet;
 public GameObject bulletInstance;
 private Rigidbody rb;
 private Rigidbody bulletRB;
 private bool isReleased = true;
 private bool m_isAxisInUse = false;
 private bool inAir = false;
 public bool isExpanding = false;
 private float bulletSizetoRecoil;

 //player control variables
 private float rTriggerRaw;
 private float moveHorizontal;
 private float rsVertical;
 private float rsHorizontal;
 private float rTrigger;
 private float xAxis;
 private float yAxis;
 private float playerNumber;
 private bool aButton;

 void Start()
 {
     //Gets the component of the player's RigidBody
     rb = GetComponent<Rigidbody>();
 }


 void FixedUpdate()
 {
     Vector3 movement = new Vector3(moveHorizontal, rb.velocity.y);
     rb.MovePosition(transform.position + (movement * playerSpeed * Time.deltaTime));
     //Jump when the player presses A 
     if (aButton && inAir == false)
     {
         rb.AddForce(0, jumpSpeed * 2f, 0, ForceMode.Force);
         inAir = true;
     }
     //Controls the aim of the gun depending on the angle of the right joystick
     if (yAxis != 0 || xAxis != 0)
     {
         float joystickAngle = Mathf.Atan2(xAxis, yAxis) * Mathf.Rad2Deg;
         pivot.transform.rotation = Quaternion.Slerp(pivot.transform.rotation, Quaternion.Euler(pivot.transform.rotation.x, pivot.transform.rotation.y, joystickAngle - 90F), Time.deltaTime * 5);
     }
     Shoot();
 }

 void Update()
 {
     if (gameObject.name == "Player 2")
     {
         rTriggerRaw = Input.GetAxisRaw("Right Trigger1");
         moveHorizontal = Input.GetAxis("Horizontal1");
         rsVertical = Input.GetAxis("Mouse Y1");
         rsHorizontal = Input.GetAxis("Mouse X1");
         rTrigger = Input.GetAxis("Right Trigger1");
         aButton = Input.GetButton("A Button1");
         xAxis = Input.GetAxis("Mouse X1");
         yAxis = Input.GetAxis("Mouse Y1");
         playerNumber = 2;
     }
     if (gameObject.name == "Player 1")
     {
         rTriggerRaw = Input.GetAxisRaw("Right Trigger");
         moveHorizontal = Input.GetAxis("Horizontal");
         rsVertical = Input.GetAxis("Mouse Y");
         rsHorizontal = Input.GetAxis("Mouse X");
         rTrigger = Input.GetAxis("Right Trigger");
         aButton = Input.GetButton("A Button");
         xAxis = Input.GetAxis("Mouse X");
         yAxis = Input.GetAxis("Mouse Y");
     }

 }
 //If thes player touches the ground then the player is no longer in the air
 void OnCollisionEnter(Collision ground)
 {
     if (ground.collider.tag == "Ground")
     {
         inAir = false;
     }
 }

 //Names the bullet depending on which player shoots the bullet
 void NameBullet()
 {
     bulletInstance.gameObject.name = "Bullet " + playerNumber;
 }

 void Shoot()
 {
     if (rTriggerRaw != 0 )
     {
         if (m_isAxisInUse == false)
         {
             bullet.transform.localScale = new Vector3(0.5f, 0.5f, 0.5f);
             bulletInstance = Instantiate(bullet, bulletEmitter.transform.position, pivot.transform.rotation);
             m_isAxisInUse = true;
         }
         bulletInstance.GetComponent<Bullet>().isExpanding = true;
         bulletInstance.transform.localScale += new Vector3(sizeIncreaseRate * Time.deltaTime, sizeIncreaseRate * Time.deltaTime, sizeIncreaseRate * Time.deltaTime);
         bulletSizetoRecoil = bulletInstance.transform.localScale.x * 2f;
         bulletInstance.GetComponent<Bullet>().bulletSizeToKnockBack = bulletInstance.transform.localScale.x * 3;
         if (bulletInstance.transform.localScale.x >= 1.25f || bulletInstance.transform.localScale.y >= 1.25f || bulletInstance.transform.localScale.z >= 1.25f)
         {
             bulletInstance.transform.localScale = new Vector3(1.25f, 1.25f, 1.25f);
         }
         if (m_isAxisInUse == true)
         {
             bulletInstance.transform.position = bulletEmitter.transform.position;
         }
     }
     if (rTriggerRaw == 0)
     {
         if (bulletInstance != null)
         {
             bulletRB = bulletInstance.GetComponent<Rigidbody>();
             if (bulletRB != null)
             {
                 isReleased = true;
                 if (bulletSizetoRecoil >= 1.1 && isReleased == true)
                 {
                     rb.AddForce(pivot.transform.right * bulletSizetoRecoil * recoil * Time.deltaTime, ForceMode.Impulse);
                     isReleased = false;
                 }
                 bulletInstance.GetComponent<Bullet>().isExpanding = false;
                 bulletRB.AddForce(pivot.transform.right * bulletSpeed, ForceMode.VelocityChange);
                 NameBullet();
                 m_isAxisInUse = false;
                 Destroy(bulletInstance, 2f);
             }
         }
     }
 }

}

Here's the code for the Bullet:

 public GameObject player;
 public GameObject pivot;
 public GameObject explosionEffect;
 private Rigidbody collisionRB;
 private Collider bulletCollider;
 public bool isExpanding = false;
 public float bulletSizeToKnockBack;
 
 void Start ()
 {
     bulletCollider = GetComponent<Collider>();
 }
 void Update()
 {
     if (isExpanding == true) {bulletCollider.enabled = false;}
     if (isExpanding == false) { bulletCollider.enabled = true;}
 }

 void OnCollisionEnter(Collision collision)
 {
     if (collision.collider.tag == ("Player") && isExpanding == false)
     {
         collisionRB = collision.collider.GetComponent<Rigidbody>();
         collisionRB.AddForce(transform.right * (100 * bulletSizeToKnockBack), ForceMode.Force);
         GameObject explosionInstance = Instantiate(explosionEffect, transform.position, transform.rotation);
         Destroy(gameObject);
         Destroy(explosionInstance, 1f);

     }
     if(collision.collider.tag == ("Ground") && isExpanding == false)
     {
         Destroy(gameObject);
     }
 }