Making a thruster shape with particles

Hi,

I'd like to know how I could make a thruster effect in Unity. I know how to create a particle system with velocity, etc... But how could I make the particle system grow smaller as it emits, just like a thruster, with an ellipsoid particle emitter?

Check out this example to see what I mean about the thruster particle system growing smaller: http://ofgabriel.com/minelander/minelander_b1/WebPlayer/

Thanks! :)