float scaler = 1;

void setup() {
  size(500,500);
  smooth();
  noStroke();
}

void draw() {

  translate(width/2,height/2); // use translate around scale
  scale(scaler);
  translate(-width/2,-height/2); // to scale from the center

  background(255);
  fill(0);
  ellipse(140,140,140,140);
  ellipse(250,200,200,200);
  ellipse(400,200,70,70);
  ellipse(230,320,20,20);
  ellipse(400,400,50,50);
  fill(255);
  ellipse(width/2,height/2,30,30);
  fill(255,0,0);
  ellipse(width/2,height/2,1,1);
}

void keyPressed() {
  if (key == 'z') {scaler -= 0.1;} // smaller
  if (key == 'x') {scaler += 0.1;} // bigger
  if (key == 'c') {scaler = 1;} // reset scale
}