Try the H.264 MKV 1080p30 preset (or any of the H.264/H.265 MKV presets). You shouldn’t have to adjust anything on the Dimensions tab for most videos. On the Video tab adjust the frame rate, quality, etc as desired.
On the Shield & Amazon Fire devices, make sure the Plex app has Display Mode = Letterbox. The setting is persistent. You do not have to check it every time you play a video.
