>>Maybe it's related to Large font setting in Windows?
>
>Bingo! That's what is causing the problem. Is there any known workarounds for this other than setting the font size back to the default?

It would be rude to change font on a user. People use Large fonts in Windows for a reason.
You may want to consider alternative DT picker, like Carlos Alloatti's date picker class and month calendar visual class from http://www.ctl32.com.ar/. I'm not sure if it has the same problem with large fonts, though.