Subtitles
 

It would be a nice feature to indicate the height of the subtitle font
in % and also to indicate different values for window mode and fullscreen
mode.

Is there a way to load vobsub's *.sub files in bsplayer ?

I'm sure that if you gave it only a try you would succeed. But if you try with v2.15 that crashes with BSPlayer, so get v2.14 or previous.

So run the vobsub install.
Give your subtitles (IDX and SUB) the same name that the AVI. You can make a RAR from your SUB, it should have the same file name too.
If you put this files and the AVI together in the same directory, loading the movie with BSPlay will autoload subtitles.