How to control size of drawn shape with controlP5

Hello, I'm new to the Processing and would love to know how to change width of the ellipse after it has been already drawn with a mouse, using slider from cp5. This is my current code which changes width of a ellipse before its been drawn with a mouse.

import controlP5.*;
ControlP5 cp5;

int slider = 100;

void setup(){
  size(800,800);
  cp5 = new ControlP5(this);
  cp5.addSlider("slider").setPosition(400,400).setRange(0,255);

}

void draw(){
  int w=slider;
  if(mousePressed) ellipse(mouseX, mouseY, w, 5);
}

Any help is appreciated. Thank you!