Issue with size() function in Processing language?
The window is 100px, 100px because it is the default size and you are not changing it, as your
size() is never being called because Setup() is not a predefined function. 
setup() is the function, lower case s not upper case S. Same with draw().

Below is the working code.


class Star {
  float x;
  float y;
  float z;

  Star(){
    // Places the star in random X and Y coordinates
    x = random(0, width);
    y = random(0, height);
  }

  /* Colors the star */
  void update(){
    fill(255);
    noStroke();
    ellipse(x, y, 8, 8);
  }

  void show(){}
  }




// Array that contains all the amount of stars created (100 in this case)
Star[] stars = new Star[100];

void setup(){
  size(800, 800); // TROUBLE HERE. DOESN´T WORK
  for(int i = 0; i &lt; stars.length; i++){
    stars[i] = new Star();
  }
}

void draw(){
  background(0); // Colors the background
  for(int i = 0; i &lt; stars.length; i++){
    stars[i].update();
    stars[i].show();
  }
}