final color  bg = color(0, 252, 210);
final static color c = #FF0000;

final static byte  i = 20; // <-- increment
int s = 20;  // <-- size

void setup() {
  size(400, 400);
  noLoop(); // deactivates draw()
  fill(c);  // paint color
} 

void draw() {
  background(bg);
  ellipse(width/2, height/2, s, s);
}

void mousePressed() {
  // increases and avoid getting bigger than width too:
  s = (s+i) % width;
  
  redraw();  // makes draw() go once
}

/**
 * Click'n'Inflate (v3.0)
 * for ArthurF (2013/Feb)
 *
 * http://forum.processing.org/topic/help-with-loops-and-mousepressed
 */

final static color[] myColors = {
  #FF0000, #00FF00, #0000FF, #FFFF00
};

final static byte[] quadrants = {
  -1, -1, -1, 1, 1, -1, 1, 1
};

final static int   numCircles = myColors.length;
final static color bg = 0100;  // <-- background color
final static byte  i = 20;     // <-- increment
int s = 20;                    // <-- size

void setup() {
  size(800, 800);   // canvas dimensions
  noLoop();         // deactivates draw()
} 

void draw() {
  background(bg);

  // Place origin (0, 0) @ center of canvas
  translate(width>>1, height>>1);  // >>1 same as /2 ;-)

  for (byte c=0; c!=numCircles; c++) {
    fill( myColors[c] );  // paint color

    ellipse( width/numCircles * quadrants[c*2], 
    height/numCircles * quadrants[c*2 + 1], s, s );
  }
}

void mousePressed() {
  // increases and avoid getting bigger than width too:
  s = (s+i) % (width>>1);

  redraw();  // makes draw() go once to refresh canvas
}