VLC HowTo/Adjust subtitle font settings
|This page describes how to change font, font size, style or colour.||Other "how to" pages|
You can with text-based subtitle formats.
Go to Tools -> Preferences (set Show Settings to All) and Video -> Subtitles/OSD -> Text renderer and adjust anything you want. Font size in pixels overrides Relative font size selection.
Remember to press Save to save VLC settings and restart VLC after that to make sure changes are enabled.