What you are you ripping those to? I would imagine either IDX/SUB or PGS?
If that's the case, then there is nothing both you or boxee can do to improve those, as dvd subtitles are standard low-res bitmap images.
If you wanted to go to the trouble, you could rip those using any OCR subtitle tool available... I'm thinking of SubRip, but I haven't used that since forever, so I don't know if there are better tools available now. Basically what it does is create an SRT file and then you would be able to control the font size, color, position within boxee itself.
Synology DS1511+ (8TB)
1x Apple TV2 with Plex and XBMC
1x Roku LT with Plex
1x Boxee Box (SOLD!)