Screen.resolutions  

function Start()
{
    var resolutions : Resolution[] = Screen.resolutions;
    // Print the resolutions
    for (var res in resolutions) {
        print(res.width + "x" + res.height);
    }
    // Switch to the lowest supported fullscreen resolution
    Screen.SetResolution (resolutions[0].width, resolutions[0].height, true);
};

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void Start() {
        Resolution[] resolutions = Screen.resolutions;
        foreach (Resolution res in resolutions) {
            print(res.width + "x" + res.height);
        }
        Screen.SetResolution(resolutions[0].width, resolutions[0].height, true);
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def Start():
        resolutions as (Resolution) = Screen.resolutions
        for res as Resolution in resolutions:
            print(((res.width + 'x') + res.height))
        Screen.SetResolution(resolutions[0].width, resolutions[0].height, true)