Screen.width

Description

The current width of the screen window in pixels (Read Only).

This is the actual width of the player window (in fullscreen it is also the current resolution).

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Example : MonoBehaviour
{
    void Start()
    {
        //Output the current screen window width in the console
        Debug.Log("Screen Width : " + Screen.width);
    }
}