This guide explains how to save screenshots from Unity at resolution of 10k+ pixels.
Download/Save As file TakeScreenshot.cs and drag it to the Project Assets / Scripts.
Open Edit menu -> Project Settings -> Quality
Disable Anti Aliasing
Drag TakeScreenshot script onto the Main Camera. Disable FPSCamera if you have one.
Start game by clicking Play button and now at any time you can press [Space] to save screenshot.
The screenshot should appear in the project folder with with a current time and date.
