I think that will be a rather difficult, if not for all practical purposes impossible, feature to accomplish.
Of course there is already the possibility to change fontsize by hand, but that is not what you mean I'm sure. You wish an automatic font-resizing or splitting of subtitle-lines.
Now, as every user is free to select what type of regular "standard" or odd and exceptional font (s)he wishes to use to display subtitles, or what size and/or weight of font ... and as most fonts are not "monospace" (even if no "kerning" is used), I can imagine it will take quite some arithmatic to figure this out for a full screen display (at different resolutions?), let alone if the movie is displayed in a freely resizable window!
In my opinion subtitles (which really are standard-procedure in my country!!) should themselves not be too long and in principle consist of one or two lines only (BSPlayer can show 3 lines).
