Serviio

I have some mp4 video files with the subtitles in a separate vtt file. The subtitles display OK when I play the video from Serviio, but they're a) in too big a font size, and b) not close enough to the bottom of the screen, so that they get in the way of viewing the action. If I play the same mp4 and vtt on my PC with VLC, then the subtitles appear in a more suitable smaller size and are positioned closer to the bottom of the screen. How can I change the size and position of the subtitles in Serviio?

I'm running Serviio 1.10.1 on Raspbian Stretch.

I'm fairly certain that your player has complete control of that. Serviio is simply sending the stream. The player has to decide how to display the information. I'm unfamiliar with a VTT file, but in an SRT file there's no ability to alter the text itself. From what I could find after a search, WebVTT files also have no support for such changes either, although it appears they do have support for Style Sheets (CSS), like an HTML page does. But that really requires a lot of effort to create those CSS pages and sync your captions file with it.

Now if you're referring to how they display when using MediaBroswer, then Zip will need to answer that question.

Depends on what device you want to play to. Under Windows, VLC is an excellent choice. Kodi is designed to be a server, but I use it as a player on my Android devices. On iOS devices, either VLC or Infuse are good. For dedicated devices like Roku, you have to use the media player they provide.