Font Size for Menus

They should follow the default size of your operating system. But you can change this under Edit → Preferences → General in the Theme section on the right hand side of the dialog.

Do check whether it is only the menu font, or also all palette items and the score view that are affected. The score should approximately match real paper size at 100% zoom.
If it doesn't match up, then it is very likely that MuseScore was unable to correctly detect your monitor resolution. You can feed it the correct information by using the -D command line option.