In my game I use
static final float FRUSTUM_WIDTH = 10;
static final float FRUSTUM_HEIGHT = 15;
So when I draw the particles they take
ParticleEffect pe = new ParticleEffect();
pe.load(Gdx.files.internal("data/particle/particle.p"), Gdx.files.internal("data/particle"));
pe.setPosition(x, y);
float pScale = 0.2f;
float scaling = pe.getEmitters().get(0).getScale().getHighMax();
pe.getEmitters().get(0).getScale().setHigh(scaling * pScale);
scaling = pe.getEmitters().get(0).getScale().getLowMax();
pe.getEmitters().get(0).getScale().setLow(scaling * pScale);
scaling = pe.getEmitters().get(0).getVelocity().getHighMax();
pe.getEmitters().get(0).getVelocity().setHigh(scaling * pScale);
scaling = pe.getEmitters().get(0).getVelocity().getLowMax();
pe.getEmitters().get(0).getVelocity().setLow(scaling * pScale);