particles.setSize

particles.setSize sets Size on a particles object. It takes id, min and max.

particles.setSize(id, min, max)

function start() {  particles.create('main', 10);  particles.setSize('main', 0, 1);}

particles.create is there to give the call something to act on; particles.setSize is the line that matters. The code sits in start(), which the engine calls once, when the entity spawns.