ETA: There's no reason to allow for very small font sizes. The only reason people need to be able to adjust font sizes is to make them larger, to compensate for failing vision, especially on large monitors.
Perhaps some users will prefer size 10 on some machines that doesn't manifest this problem? Is this really much of a problem? The default is fine so people fiddling with the font size will likely understand they need to play with that setting again if things starts looking bad.