let rectExpand = 1.05;

​

function setup() {

  createCanvas(400, 400);

  rectMode(CENTER);

  noFill();

}

​

function draw() {

  background(255);

  line(0, 0, width, height);

  line(width, 0, 0, height);

  rect(width/2, height/2, rectSize, rectSize);

  rectSize = rectSize * rectExpand;

  if(rectSize >= width) {

     rectSize = 1; 

  }

}