Create a grid on a cube?

Hello! :) I'm working on a game which needs a grid. I have a 10x10 flat cube which I use as my "grid area", what I want to do is creating a grid inside this cube which resizes according to a parameter. For example if I want a 5x5 grid, the grid area will consist of 25 cubes of the same size (like a normal grid d'oh).

I have a simple script that creates a grid and instatiates objects according to the grid: var prefabCount=0; var tGridX=0; var tGridY=0; tubesArray=new GameObject[gridY,gridX];

 var posY=-1.0;
 for (var y= 0; y< gridY; y++){
     
     for (var x=0; x<gridX; x++){
         selectPrefab(gridChars[prefabCount]);
         var pos=Vector3(x, posY, 0)*spacing;
         var temp= Instantiate(myPrefab, pos, Quaternion.identity);
         tubesArray[y,x]=temp;
         tubesArray[y,x].GetComponent(CubeProperties).setPosition(y,x);
         prefabCount++;
         
         }
     posY=posY+1.0; 
     }
     
 }

This works but I have to set Y and X positions and prefabs sizes to fit the screen (it's an Android project btw). I need a better grid system which allows me to change the size of the prefabs I have to instantiate, to fit the square tile size of the grid without changing it manually everytime.

It's way more difficult to explain than I tought :( I tought I could do just like in real life where i just take the size of the area (10x10) and divide it by 3 if i want a 3x3 grid; so each grid's tile would be of around 3,33, and set the prefab size to that; but it doesn't work D:

Any suggestions? Solving this problem would boost my project a lot!

PS. Forgot to say that the prefabs are just square sized gameobects sprites, don't know it is useful