1. void setup()
   {
     size(500, 500);
     smooth();
     noStroke();
   }
   
   void draw()
   {
     background (255);
     fill (0, 0, 255);
    
     float radius = 10;
     float xpos = width / 2;
     float ypos = height / 2;
   
     if(mouseButton == RIGHT)
     {
       radius = map(mouseX, 0, width, 10, 200);
     }
     if(mouseButton == LEFT)
     {
       xpos = mouseX;
       ypos = mouseY;
     }
     ellipse(xpos, ypos, radius, radius);
   }

thanks for code. problem is that once you click right it goes to middle, i' d love to know how to keep it in its current dragged position and then change its size...

Here is the basic functionality you talk about. Of course one can add complexity by checking if the mouse is within the radius, using easing for the circle to move towards the mouse and - when resizing the radius - checking if the mouse is moving towards or away from the center of the circle to decrease/increase the radius.

Code Example

1. float x, y;
2. float radius = 50;
3. float tempRadius;
4.  
5. void setup() {
6.   size(500, 500);
7.   smooth();
8.   noStroke();
9.   fill(0, 0, 255);
10. }
11.  
12. void draw() {
13.   background(255);
14.   ellipse(x, y, radius, radius);
15. }
16.  
17. void mousePressed() {
18.   if (mouseButton == LEFT) {
19.     x = mouseX;
20.     y = mouseY;
21.   } else if (mouseButton == RIGHT) {
22.     tempRadius = radius;
23.   }
24. }
25.  
26. void mouseDragged() {
27.   if (mouseButton == LEFT) {
28.     x = mouseX;
29.     y = mouseY;
30.   } else if (mouseButton == RIGHT) {
31.     radius = tempRadius += (mouseX-pmouseX);
32.     radius = max(radius,0);
33.   }
34. }