Editor only script that enables adding of bespoke Game resolutions to the GameView dropdown selector. Support to create landscape and portrait resolutions, different resolutions for different platforms etc.

GameViewManager.cs

// Based on
// http://answers.unity3d.com/questions/956123/add-and-select-game-view-resolution.html

using System;
using UnityEditor;
using UnityEngine;

namespace NoiseCrime.Editor.Tools
{
	public static class GameViewAddResolutions
	{
		public enum GameViewSizeType { AspectRatio, FixedResolution }

		[MenuItem( "Window/NoiseCrimeMethods/GameViewResolutions/RemoveAddAllPlatforms" )]
		public static void RemoveThenAddAllPlatformSizes()
		{
			GameViewResolutionsRemoveAll();

			GameViewResolutionsStandalone();
			GameViewResolutionsIOS();
			GameViewResolutionsAndroid();
		}

		[MenuItem( "Window/NoiseCrimeMethods/GameViewResolutions/AddStandalone" )]
		public static void GameViewResolutionsStandalone()
		{
			AddDesktopFixedResolutionsToGroup(GameViewSizeGroupType.Standalone);	
			AddMobileFixedResolutionsToGroup(GameViewSizeGroupType.Standalone);	
			SaveCustomSizes();
		}

		[MenuItem( "Window/NoiseCrimeMethods/GameViewResolutions/AddIOS" )]
		public static void GameViewResolutionsIOS()
		{
			AddMobileFixedResolutionsToGroup(GameViewSizeGroupType.iOS);	
			SaveCustomSizes();
		}

		[MenuItem( "Window/NoiseCrimeMethods/GameViewResolutions/AddAndroid" )]
		public static void GameViewResolutionsAndroid()
		{
			AddMobileFixedResolutionsToGroup(GameViewSizeGroupType.Android);
			SaveCustomSizes();	
		}

		[MenuItem( "Window/NoiseCrimeMethods/GameViewResolutions/RemoveAll" )]
		public static void GameViewResolutionsRemoveAll()
		{
			RemoveCustomSizes(GameViewSizeGroupType.Standalone);
			RemoveCustomSizes(GameViewSizeGroupType.iOS);
			RemoveCustomSizes(GameViewSizeGroupType.Android);
			SaveCustomSizes();
		}


		static void AddDesktopFixedResolutionsToGroup( GameViewSizeGroupType sizeGroupType )
		{
			AddFixedResolutionDual( sizeGroupType, 1280,  720, "HDR" );
			AddFixedResolutionDual( sizeGroupType, 1920, 1080, "HD" );

			AddCustomSize( GameViewSizeType.FixedResolution, sizeGroupType, 3440, 1440, "UWHD Landscape" );
			AddCustomSize( GameViewSizeType.FixedResolution, sizeGroupType, 1024, 1024, "Square" );	
			AddCustomSize( GameViewSizeType.FixedResolution, sizeGroupType, 1201, 1464, "Oculus Rift" );	
			
			// Custom Clients
			AddCustomSize( GameViewSizeType.FixedResolution, sizeGroupType, 3840, 1200, "CTFull" );
			AddCustomSize( GameViewSizeType.FixedResolution, sizeGroupType, 1920,  600, "CTHalf" );			
		}


		static void AddMobileFixedResolutionsToGroup( GameViewSizeGroupType sizeGroupType )
		{
			AddFixedResolutionDual( sizeGroupType, 1280,  800, "Mobile" );
			AddFixedResolutionDual( sizeGroupType,  960,  540, "Lumia 535" );
			AddFixedResolutionDual( sizeGroupType, 1920, 1200, "Nexus 7" );
			AddFixedResolutionDual( sizeGroupType, 1280,  720, "Moto G" );

			AddFixedResolutionDual( sizeGroupType, 1024,  768, "iPad 2" );
			AddFixedResolutionDual( sizeGroupType, 2048, 1536, "iPad Pro 9.7" );
			AddFixedResolutionDual( sizeGroupType, 2732, 2048, "iPad Pro 12.9" );

			AddFixedResolutionDual( sizeGroupType,  480,  320, "iPhone" );
			AddFixedResolutionDual( sizeGroupType,  960,  640, "iPhone 4" );
			AddFixedResolutionDual( sizeGroupType, 1136,  640, "iPhone 5" );
			AddFixedResolutionDual( sizeGroupType, 1334,  750, "iPhone 6" );
			AddFixedResolutionDual( sizeGroupType, 1920, 1080, "iPhone 6+" ); // 1242 x 2208 downsampled
			AddFixedResolutionDual( sizeGroupType, 2208, 1242, "iPhone 5.5" ); // 1242 x 2208 downsampled
		}

		static void AddFixedResolutionDual( GameViewSizeGroupType sizeGroupType, int width, int height, string text )
		{
			AddCustomSize( GameViewSizeType.FixedResolution, sizeGroupType, width,  height, text + " Landscape" );
			AddCustomSize( GameViewSizeType.FixedResolution, sizeGroupType, height,  width, text + " Portrait" );
		}

		static void SaveCustomSizes()
		{
			var asm					= typeof(UnityEditor.Editor).Assembly;
			var sizesType			= asm.GetType("UnityEditor.GameViewSizes");
			var singleType			= typeof(ScriptableSingleton<>).MakeGenericType(sizesType);
			var instanceProp		= singleType.GetProperty("instance");
			var saveToHDD			= sizesType.GetMethod("SaveToHDD");
			var instance			= instanceProp.GetValue(null, null);

			saveToHDD.Invoke(instance, null);			
		}

		static void AddCustomSize( GameViewSizeType viewSizeType, GameViewSizeGroupType sizeGroupType, int width, int height, string text )
		{
			// goal:
			// var group = ScriptableSingleton<GameViewSizes>.instance.GetGroup(sizeGroupType);
			// group.AddCustomSize(new GameViewSize(viewSizeType, width, height, text);

			var asm				= typeof(UnityEditor.Editor).Assembly;
			var sizesType		= asm.GetType("UnityEditor.GameViewSizes");
			var singleType		= typeof(ScriptableSingleton<>).MakeGenericType(sizesType);
			var instanceProp	= singleType.GetProperty("instance");
			var getGroup		= sizesType.GetMethod("GetGroup");
			var instance		= instanceProp.GetValue(null, null);
			var group			= getGroup.Invoke(instance, new object[] { (int)sizeGroupType });

			var addCustomSize	= getGroup.ReturnType.GetMethod("AddCustomSize"); // or group.GetType().
			var gvsType			= asm.GetType("UnityEditor.GameViewSize");
			var ctor			= gvsType.GetConstructor(new Type[] { typeof(int), typeof(int), typeof(int), typeof(string) });
			var newSize			= ctor.Invoke(new object[] { (int)viewSizeType, width, height, text });
			addCustomSize.Invoke( group, new object[] { newSize } );
		}

		static void RemoveCustomSizes( GameViewSizeGroupType sizeGroupType )
		{
			var asm					= typeof(UnityEditor.Editor).Assembly;
			var sizesType			= asm.GetType("UnityEditor.GameViewSizes");
			var singleType			= typeof(ScriptableSingleton<>).MakeGenericType(sizesType);
			var instanceProp		= singleType.GetProperty("instance");
			var getGroup			= sizesType.GetMethod("GetGroup");
			var instance			= instanceProp.GetValue(null, null);
			var group				= getGroup.Invoke(instance, new object[] { (int)sizeGroupType });

			var getCustomCount		= getGroup.ReturnType.GetMethod("GetCustomCount"); // or group.GetType(). 
			var getBuiltinCount		= getGroup.ReturnType.GetMethod("GetBuiltinCount"); // or group.GetType(). 
			var removeCustomSize	= getGroup.ReturnType.GetMethod("RemoveCustomSize"); // or group.GetType(). 

			int customCount			= (int) getCustomCount.Invoke(group, null);
			int builtinCount		= (int) getBuiltinCount.Invoke(group, null);


			Debug.Log("RemoveCustomSizes for Group: " + sizeGroupType.ToString() + "  Count: " + customCount + "  BuiltIn: " + builtinCount);

			for ( int i = 0; i < customCount; i++ )
			{
				removeCustomSize.Invoke( group, new object[] { builtinCount + (customCount - i - 1) } );
			}
		}
	}
}