ParticleSystem.MainModule.startSizeZMultiplier

Description

The initial size multiplier of particles along the Z axis when emitted.

This method is more efficient than accessing the whole curve, if you only want to change the overall size multiplier.

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour
{
    private ParticleSystem ps;
    public float hSliderValue = 1.0F;

    void Start()
    {
        ps = GetComponent<ParticleSystem>();

        var main = ps.main;
        main.startSize3D = true;

        var psr = GetComponent<ParticleSystemRenderer>();
        psr.renderMode = ParticleSystemRenderMode.Mesh;
        psr.mesh = Resources.GetBuiltinResource<Mesh>("Cube.fbx");// use a mesh, because billboards have no Z axis size
        psr.material = new Material(Shader.Find("Sprites/Default"));
    }

    void Update()
    {
        var main = ps.main;
        main.startSizeZMultiplier = hSliderValue;
    }

    void OnGUI()
    {
        hSliderValue = GUI.HorizontalSlider(new Rect(25, 45, 100, 30), hSliderValue, 0.0F, 10.0F);
    }
}