sorry but I have no idea why your font looks smaller at the lower left position - maybe Zef or some of the other guys could enlighten us on this issue...?!
Thin fonts in low pt values however tend to lose saturation when being rendered to pixels. This is actually nothing I could possibly "fix", since the antialiasing of the individual pixels is controlled from within gdlib and nothing I can affect at all (at least as of my current knowledge) - sorry pal!
Regarding your requests: I have added pt values 4-7 to the size selector - even though I doubt it will produce anything fairly usable... At the given 72dpi picture resolution in the resulting PNG, 4pt sized fonts will most likely be blurry and unreadable. Furthermore I added three negative char spacing options - but using these will BREAK YOUR FONTFILES in most cases... Anyway - have fun!
The preview will definitely have background color selection, since it would be no "valid preview" without such an option...