Paul,

 When you save a mosaic image as a bitmap, the resolution is optimized for resolution of your MRI scan. The idea is to minimize interpolation artifacts. The font size is intentionally disregarded, and uses a rough algorithm based on minimum image dimension (so with long thin single row mosaics height becomes a constraint). I would suggest you save two copies of your mosaic - one with labels and one without. You can use the one without labels for any publications - either report the slice positions in the text legend or use a vector art editor to add high quality text lebels where you want them. 

In this respect, the mosaic view is different from the other views, where bitmaps are optimized for screen resolution at the expense of interpolation artifacts. 

Note you can go to Preferences and increase the bitmap zoom. This will apply an integer scale factor to the whole image. This often benefits the appearance of text labels on the MRI bitmaps, though obviously it creates larger files and does introduce subtle artifacts to the images.

In brief, this is a feature not a weakness of my software. There is a tradeoff between text quality and preserving the raw resolution of your images. When you save a mosaic the software is protecting the quality of your MRI scan at the cost of your text.