[r0lZ]

No. Normally, the size of the graphics subtitles is determined by the author, and cannot be changed. It's a important advantage for the 3D subtitles, as otherwise, if the player shrinks (or expands) them, the offset between the two views will change, and therefore the subtitles will appear at the wrong depth. That can and certainly will ruin the 3D experience.

Of course, the problem is different for 2D subtitles, and it may be more pleasant to reduce them somewhat, especially if the movie is played on a computer. You can of course demux them, use the scale feature of BDSup2Sub, and remux the modified subtitle in the MKV, but AFAIK you cannot do that on the fly. And of course, I don't recommend to do it with the 3D subtitles.