Remove the letterbox when switching windows mode to full screen mode, set the windows to manual screen width when switching full screen mode to windows mode. Standalone only.

ScreenModeSwitcher.cs

using UnityEngine;

namespace SweatyChair
{

	/// <summary>
	/// Remove the letterbox when switching windows mode to full screen mode, set the windows to manual screen width (if
	/// speicified) when switching full screen mode to windows mode. Standalone only.
	/// </summary>
	public class ScreenModeSwitcher : PersistentSingleton<ScreenModeSwitcher>
	{

#if UNITY_STANDALONE

		private const string PREFS_FULL_SCREEN_WIDTH = "FullScreenWidth";
		private const string PREFS_FULL_SCREEN_HEIGHT = "FullScreenHeight";

		private bool _fullScreen;

		private void Start()
		{
			_fullScreen = Screen.fullScreen;
		}

		private void Update()
		{
			if (_fullScreen != Screen.fullScreen) {
				if (_fullScreen) { // Full screen mode to window mode
					Debug.LogFormat("ScreenModeSwitch - Full screen mode changed to windows mode, Screen.width={0}, Screen.height={1}, Screen.currentResolution={2}", Screen.width, Screen.height, Screen.currentResolution);
					// Set resolution with manual width if specified
					if (SettingSettings.current.manualScreenWidth > 0)
						Screen.SetResolution(SettingSettings.current.manualScreenWidth, Mathf.RoundToInt(1f * SettingSettings.current.manualScreenWidth * Screen.height / Screen.width), Screen.fullScreen);
					// Save the full screen ratio, because this will be overwritten in window mode if player resize the windows
					PlayerPrefs.SetInt(PREFS_FULL_SCREEN_WIDTH, Screen.width); // Save the screen ratio, so this can be used next time full screen
					PlayerPrefs.SetInt(PREFS_FULL_SCREEN_HEIGHT, Screen.height); // Save the screen ratio, so this can be used next time full screen
				} else { // Windows mode to full screen mode
					Debug.LogFormat("ScreenModeSwitch - Windows mode changed to full screen mode, Screen.width={0}, Screen.height={1}, Screen.currentResolution={2}", Screen.width, Screen.height, Screen.currentResolution);
					// Both Screen.width/height and Screen.currentResolution are set to the windows size, try get the full screen ratio from saved PlayerPrefs value or default it to 16:9
					int fullScreenWidth = PlayerPrefs.GetInt(PREFS_FULL_SCREEN_WIDTH, Screen.width);
					int fullScreenHeight = PlayerPrefs.GetInt(PREFS_FULL_SCREEN_HEIGHT, Screen.height);
					Debug.LogFormat("ScreenModeSwitch - fullScreenWidth={0}, fullScreenHeight={1}", fullScreenWidth, fullScreenHeight);
					if (SettingSettings.current.manualScreenWidth > 0) // Set to resolution relative to mannual width
						Screen.SetResolution(SettingSettings.current.manualScreenWidth, Mathf.RoundToInt(1f * SettingSettings.current.manualScreenWidth  * fullScreenHeight / fullScreenWidth), Screen.fullScreen);
					else // Set to resolution relative to current width
						Screen.SetResolution(fullScreenWidth, fullScreenHeight, Screen.fullScreen);
				}
				_fullScreen = Screen.fullScreen;
			}
		}

#endif

	}

}