float x = 0;
float y = 0;
float s = 30;

void setup() {
  size(800, 800);
  background(255);
  fill(0);
  smooth();
  rectMode(CENTER);
  noStroke();
}

void draw() {

  if (random(10) > 9) {
    s = 10;
  }
  else {
    s = 30;
  }
  
  rect(x, y, s, s);
  

  x += 50;
  if (x > width) {
    x = 0;
    y += 50;
  }
  
}