HoloLens game window size in Unity editor.

Unity’s game view control bar allows you to choose aspects (sizes). The aspect dropdown lists the available sizes:

The HoloLens resolution

According to the HoloLens documentation, the default and maximum supported resolution is 720p (1268x720). To add that resolution,

• click the aspect dropdown from the game view bar
• click the plus button:

• enter label HoloLens
• enter width 1268
• enter height 720

• click OK

The game view now better reflects what you will see when deploying to the HoloLens.