I consider subtitle text rendering is part of the player feature. If you are using VLC, may this link, https://wiki.videolan.org/VLC_HowTo/Adjust_subtitle_font_settings/, help?
I had tried on a mkv to mp4 video, before
After, I changed the VLC subtitle font size to larger, exited the program and replayed it again, i got
To use newlines in the built-in Subtitle filter, you must specify the lsp (linespacing) argument:
subtitle("First line\nNext line\nLast line", lsp=12)
See the Subtitle wiki page for more info on the argument.