I'm just trying to export a single frame of the video... here are my settings:
Not sure why, but it's exporting the length and width 2 pixels larger than the video dimension. So a 1920x1080 video is exporting an image as 1922x1082.
If you want just a single native sized frame - please use "Output each tile to a separate file" feature in pair with "Keep original frame size" (Environment window, hotkey: F3).