ParticleSystem.SizeBySpeedModule.xMultiplier

Description

Size multiplier along the x-axis.

Changing this property is more efficient than accessing the entire 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 hSliderValueX = 1.0f;
    public float hSliderValueY = 1.0f;
    public float hSliderValueZ = 1.0f;

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

        var sizeBySpeed = ps.sizeBySpeed;
        sizeBySpeed.enabled = true;
        sizeBySpeed.separateAxes = true;

        AnimationCurve curve = new AnimationCurve();
        curve.AddKey(0.0f, 0.0f);
        curve.AddKey(1.0f, 1.0f);

        ParticleSystem.MinMaxCurve minMaxCurve = new ParticleSystem.MinMaxCurve(1.0f, curve);

        sizeBySpeed.x = minMaxCurve;
        sizeBySpeed.y = minMaxCurve;
        sizeBySpeed.z = minMaxCurve;
        sizeBySpeed.range = new Vector2(1.0f, 10.0f);

        var psr = GetComponent<ParticleSystemRenderer>();
        psr.material = new Material(Shader.Find("Sprites/Default"));    // this material renders a square billboard, making it easier to see the size
    }

    void Update()
    {
        var sizeBySpeed = ps.sizeBySpeed;
        sizeBySpeed.xMultiplier = hSliderValueX;
        sizeBySpeed.yMultiplier = hSliderValueY;
        sizeBySpeed.zMultiplier = hSliderValueZ;
    }

    void OnGUI()
    {
        GUI.Label(new Rect(25, 40, 100, 30), "X");
        GUI.Label(new Rect(25, 80, 100, 30), "Y");
        GUI.Label(new Rect(25, 120, 100, 30), "Z");

        hSliderValueX = GUI.HorizontalSlider(new Rect(55, 45, 100, 30), hSliderValueX, 1.0f, 5.0f);
        hSliderValueY = GUI.HorizontalSlider(new Rect(55, 85, 100, 30), hSliderValueY, 1.0f, 5.0f);
        hSliderValueZ = GUI.HorizontalSlider(new Rect(55, 125, 100, 30), hSliderValueZ, 1.0f, 5.0f);
    }
}