Some web players use a 920x516 resolution. This maintains a roughly 16:9 aspect ratio suitable for many online platforms.
To change a file (like an MKV) into an MP4, use VLC Media Player by selecting Media > Convert/Save and choosing the Video - H.264 + MP3 (MP4) profile. 516 mp4
If you are converting physical film to MP4 using a device like the KODAK REELS Film Digitizer Go to product viewer dialog for this item. Some web players use a 920x516 resolution