I’m struggling with this as well.
I followed this tip (UE4 + HTC Vive Fullscreen VR Tutorial - YouTube) to get the desktop preview to be 16x9, but it still starts fullscreen, and if I alt-enter to go windowed, the window is huge, and bleeds off screen.
I’d REALLY like to be able to render that cropped image in a window at that native res, or at least have a way I can provide the window size so I can tweak it myself.
Is this possible? I’ve tried playing with console commands and command line args, but haven’t had any luck. r.setres 1280x720 gives me a 1280x720 window like I want, but breaks VR completely 
Thanks!
UPDATE:
I found I could achieve what I wanted by editting line 1026 of UE4/Plugins/Runtime/Steam/SteamVR/Source/SteamVR/Private/SteamVRHMD.cpp to read:
FSystemResolution::RequestResolutionChange(1600, 900, EWindowMode::Windowed);