Screen.resolutions  

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 {
public Resolution[] resolutions = Screen.resolutions;
void Awake() {
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):

public resolutions as (Resolution) = Screen.resolutions

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