Each MFD selects its own font when it renders, so the ability to configure a given MFD's font depends on whether that MFD's code supports that. I don't know of any MFDs that support configurable fonts, but it's possible that some MFDs, somewhere, do. You could check the documentation for each MFD that you're interested in updating.
If you primarily want to change the font size on all MFDs, you could play around with the "Panel Scale" setting in the Orbiter Launchpad: that will scale the size of all the 2D panels, though, not just the MFDs. But it's useful if you're playing on a high-resolution monitor and want to make the 2D panels fit the width of the screen. :tiphat: