ok you need to do this first:

create a creation expression first (in the same way you made a runtime one) but do this:

//this is your start value
particleShape2.radiusPP == 0.05;

then change your runtime expression to something like this:

//get the value of the radiusPP
float $raggio = particleShape1.radiusPP;

//compare it to the max value you want them to grow to
if($raggio < 1)
{
//add a value on this can be anything you want it to be
$raggio+=0.001;
//set the value of the radiusPP
particleShape1.radiusPP=$raggio;
}

That should work.

:bandit:
Alan