The font constants (FONT_XTINY, FONT_TINY, FONT_SMALL, FONT_MEDIUM, FONT_LARGE) are useful, but they don't cover all of the available font sizes. If you extend SimpleDataField, you see a huge font used for displaying numeric values. Even the FONT_LARGE font isn't as big as the font used by the provided data field.
I did some testing and it appears that font values map to integer constants with the values FONT_XTINY = 1, FONT_TINY = 2, FONT_SMALL = 3, FONT_MEDIUM = 4, FONT_LARGE = 5. If I use integer values larger than 5 and less than or equal to 13, I get properly rendered results for numeric values (alpha characters render as diamonds).
I'm assuming this is by design, but it would be useful if the fact was documented. It might also be worthwhile to add constants for fonts 5-13.