To access the Preferences window, select File > Preferences.
(On a Mac, choose Moneydance > Preferences).
We always recommend restarting Moneydance after making a change within the Preferences window, to ensure the change has been fully applied.
Moneydance 2021 and newer versions
The preferences window includes the six tabs displayed below -

You'll find information about each preferences tab within this section of the knowledge base.
Moneydance 2020 and older versions
The preferences window includes the six tabs displayed below -

You'll find the settings relating to "Appearance" (font size and theme) within the "General" tab.