Rebuild Roguelike with other sprite sheet size

I follow the Roguelike tutorial series. I've trying to rebuild it my own to learn most of it.

However I'm at the "BoardManager" now and instead of a nice board like in the tutorial I get something kind of spread tiles.

I see that my sprite sheet is 1024x1024 instead of 256x256 like in the tutorial. How can I tell Unity to expand my sprite so they follow each other?

Here my BoardManager script:

 using UnityEngine;
 using System;
 using System.Collections.Generic;       //Allows us to use Lists.
 using Random = UnityEngine.Random;      //Tells Random to use the Unity Engine random number generator.
 
 public class BoardManager : MonoBehaviour {
 
 
     public int columns = 8;
     public int rows = 20;
     public GameObject[] backgroudTiles;
     public GameObject[] undergroundTiles;
 
     private Transform boardHolder;
     private List<Vector3> gridPositions = new List<Vector3>();
 
     private void InitializeList()
     {
         gridPositions.Clear();
         for (int x = 0; x < columns; x++) {
             for (int y = 0; y < rows; y++) {
                 gridPositions.Add(new Vector3(x,y,0f));
             }
         }
     }
 
     private void BoardSetup()
     {
         boardHolder = new GameObject("Board").transform;
 
         for (int x = 0; x < columns; x++) {
             for (int y = 0; y < rows; y++) {
                 // Background tiles
 
                 // randomly choose a background tile
                 GameObject toInstantiate = backgroudTiles[Random.Range(0, backgroudTiles.Length)];
                 // instantiace it
                 GameObject instance = Instantiate(toInstantiate, new Vector3(x,y,0f), Quaternion.identity) as GameObject; 
                 // parent it to the board holder
                 instance.transform.SetParent(boardHolder);
             }
         }
     }
 
     private Vector3 RandomPosition()
     {
         int randomIndex = Random.Range(0, gridPositions.Count);
         Vector3 randomPosition = gridPositions[randomIndex];
         gridPositions.RemoveAt(randomIndex);
         return randomPosition;
     }
 
     //SetupScene initializes our level and calls the previous functions to lay out the game board
     public void SetupScene (int level)
     {
         BoardSetup();
         InitializeList();
     }
 
         // Use this for initialization
     void Start () {
     
     }
     
     // Update is called once per frame
     void Update () {
     
     }
 }