Save a screenshot in any resolution from the Unity Editor. Useful for making screenshots higher than your monitor's resolution.

Screenshotter.cs

using System;
using System.IO;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using UnityEngine;
using UnityEditor;

/*
	AUTHOR: Stijn Raaijmakers @bugshake
	
	Custom editor window which will save a screenshot in the play windows resolution.
	For example if you have a 1080p screen, but want a 4k screenshot, just set that resolution in the Game window
	and the file will be a full 3840x2160 pixels
*/
public class Screenshotter : EditorWindow
{
	List<string> fileHistory = new List<string>();
	Vector2 scrollPos = Vector2.zero;

	[MenuItem("Tools/Screenshotter...")]
	public static void ShowWindow()
	{
		var win = EditorWindow.GetWindow<Screenshotter>(false, "Screenshotter");
	}

	void OnGUI()
	{
		string screenshotPath = string.Format("{0}/Screenshots", Application.persistentDataPath);
		if (GUILayout.Button("Capture"))
		{
			Directory.CreateDirectory(screenshotPath);
			string path = string.Format("{0}/s_{1:yyyy_MM_dd hh_mm_ss}.png", screenshotPath, DateTime.Now);
			Application.CaptureScreenshot(path);
			fileHistory.Add(path);
		}
		if (GUILayout.Button("SCREENSHOTS:"))
		{
			Directory.CreateDirectory(screenshotPath);
			EditorUtility.RevealInFinder(screenshotPath + "/");
		}
		scrollPos = GUILayout.BeginScrollView(scrollPos, GUILayout.Height(100f));
		for( int i = fileHistory.Count; i-- > 0;)
		{
			GUILayout.Label(fileHistory[i]);
		}
		GUILayout.EndScrollView();
	}
}