ParticleStartSizeTween.cs

using DG.Tweening;
using UnityEngine;

[RequireComponent(typeof(ParticleSystem))]
public class ParticleStartSizeTween : MonoBehaviour
{
    void Start()
    {
        ParticleSystem particle = this.GetComponent<ParticleSystem>();

        float toSize = 1.0f;
        float duration = 10.0f;

        DOTween.To(
            () => particle.startSize,      // 対象の値を指定
            it => particle.startSize = it, // 変化するitを代入
            toSize,                        // 最終値
            duration                       // 時間
        ).Play();
    }
}