There is an option in Subtitles Preferences that accomplish your request.
Open Preferences window (CTRL+P) then go to Subtitles section and select the option "Auto resize font if line is too long".
Note: this option doesn't work for you if your subtitle has very short sentences since BSplayer resizes the subtitle lines only if they exceed MovieWindow width.
As an alternative you can change the subtitle font size still in Subtitle Preferences by pressing the Font button in up-right corner!
Bye
