No problem! You just need to fire up XBMC and navigate to Settings -> Videos -> Subtitles; There you'll be able to change a bunch of subtitle presentation variables including the font, size of the font, style (normal, bold, italics), the color, which character set (if you needed special characters for a language like Farsi or such) and whether or not you want your subtitles options to override the plaintext fonts that are embedded in ASS/SSA compliant media.
The last option is setting the subtitle locaton. You can set it to Fixed (the default) which keeps the subtitle bar where you set it in the video calibration screen (go to Settings -> System -> Video Output -> Video Calibration... to fine tune this if the subtitles are too close to the bottom/edge of the screen). Otherwise you can specify above/below (for video that doesn't fill the screen, which is rare) or top/bottom (to orient it at the very top or very bottom). I'd stick with adjusting it yourself using the Video Calibration to put it right where you want. Hope that helps!
If you need further help with subtitles or calibration, check out the following XBMC wiki links: