fill screen.

FillScreen.cs

using UnityEngine;

namespace Game
{
    public class FillScreen : MonoBehaviour
    {
        public SpriteRenderer sr;

        private void Awake()
        {
            var screenRadio = (float) Screen.height / Screen.width;
            var spriteRadio = sr.size.y / sr.size.x;
            if (screenRadio > spriteRadio)
            {
                sr.size = screenRadio / spriteRadio * sr.size;
            }
        }
    }
}