It's not *that* simple unless you have a limited number of circles.  Each frame, you need to know:
1. where each circle is (x,y)
2. how large each circle is. (r)

There is a class, PVector, which can take 3 variables (intended to be used for x,y, and z, but you could use z for r).  If you don't want to learn to write classes, that is a possible approach.  (An array of PVectors.)

However, ideally you'd have a 'circle' class, instances of which are generated on click, and put into a list with a flexible number of elements, like an ArrayList or Vector.  If you're using Vectors, the syntax looks like this:

Vector circleList = new Vector();
void mousePressed(){ circleList.add(new circle()); }
for (int i=0;i<circleList.size();i++){
  ((circle)circleList.get(i)).expand();
  ((circle)circleList.get(i)).render();
}

etc.

Then a circle class, which could look something like this:

class circle{
 float x,y,r;
 circle(){
    x = mouseX;
    y = mouseY;
    r = 1;
 }
 void expand(){
    r++;
 }
 void render(){
    etc.
 }
}

Hope this is clear
--Ben