i'd say you should do some reading on how classes (and objects) works, your idea can be done using just one array filled with objects. Also, now that i think about it, you could just use a PVector instead of making your own particle class only then all particles must share a common size and color (you won't be able to store it in the same PVector so if you want that you still need to create your own particle class)

What is there to explain? Assuming that all particles have the same size and color (and the common size and color is depending on the mouse x and y), all you need to do is to store the particles positions.

(having typed all there i found a useful link. read this first: http://processing.org/learning/objects/ )

anyway:

define an array of particles at the top:

Particle[] particles = new Particle[NUMBER_OF_PARTICLES];

your setup should be something like:

// instatiate
for (int i = 0 ; i < NUMBER_OF_PARTICLES ; i++) {
  particle[i] = new Particle();
}

and your draw something like
// update and draw all particles
for (int i = 0 ; i < NUMBER_OF_PARTICLES ; i++) {
  particle[i].update();
}
for (int i = 0 ; i < NUMBER_OF_PARTICLES ; i++) {
  particle[i].draw();
}

and there should be a Particle class

class Particle {
// member variables here

  // constructor
  Particle() {
    init();
  }

  void init() {
// initialise members here
  }

  void update() {
// change this particle here
  }

  void draw() {
// draw this particle here
  }
}

the class is the actual object definition. all your particles are objects. they know only what you tell them. they know how to update themselves and how to display themselves. this is basically the bones of object-oriented languages.

sounds like each of your particles needs to know:
current x position
current y position
its velocity (x only at the moment, but define y as well, just in case)
its size
its colour

so pick some names, pick some types, add them as member variables to your class. think of how you're going to initialise them. and how you're going to update them. and how you're going to draw them. start with one or two, watch what they do. tweak.

(i write this code so often i oughta keep a basic copy of it around just to save me typing it every time)