I spoke with cp15 on Navit IRC the other night and brought up the issue regarding the max font size of 15. He fixed the issue and as of SVN 2311 you should now be able to specify up to a maximum font size of 48.

I currently have most street names a font size of 20 and it looks pretty good with Navit. The next thing I need to figure out is how to specify the font type and perhaps to make it bold.