How to get resolution of app video preview for iPhone 6 (750 x 1334)?

1) Download HandBrake (open source video transcoder)

2) Open HandBrake, click picture settings in the top right and enter 750 x 1334 for the resolution and close the window.

3) Change the anamorphic setting to none in picture settings (or else it will steal one of your pixels and make it 1333 x 750).

4) Click start in the upper left of the main window and it will save your new video to the desktop.

Edit: Updated to reflect fix mentioned below to prevent losing one pixel on the width (thank you).

As @quemeful mention solution above can give you 1333x750 video. To get strict 1334x750 resolution use anamorphic: none in picture settings.