Font size in VMWare Fusion on my Mac
It sounds like a resolution issue, more than a font size issue. If so, take these steps:

You may have to shut down (not suspend) your VM whilst making these changes.

Open the Virtual Machine Library window (Shift + Command + L)

On the left side of the window, select the VM you want to change the resolution.

At the top of the Library window select "Settings"

Under Settings, choose "Display"

On the bottom half of the Display preference pane, uncheck "Use full resolution for Retina Display".

If this does not resolve your issue completely, you may need to adjust the display resolution within your VM was well using it's own display preferences.