Hi Marnine,
That is odd. While you can control the general font size in the UI via the Preferences, I'm not familiar with any option to change the type of font used by the Channel Box alone. Have you tried deleting your preferences and starting Maya clean? If that doesn't work, then the only other thing I can think of is that it has something to do with the type of fonts used by your Operating System. I do know that Maya draws from your OS settings to determine its fonts. It could also be an issue with the resolution of your monitor or your GPU. Try updating your drivers or changing the resolution of your monitor just to see if that changes.