Make sure you don't have Wurm Unlimited running.

If you have a shortcut for the Wurm Client executable, right click and select Properties. Otherwise, navigate to the WurmLauncher folder and find WurmLauncher.exe, then right click and select Properties.

In the Properties change to the Compatibility tab.

Click the Change High DPI Settings button.

Check both checkboxes on the High DPI Settings window and set the lower drop box to Settings.

Click Ok on both windows to accept the new settings.

Open the Wurm Client (WurmLauncher.exe) and click on the settings button (the gear). Modify font settings. Load into the game and test. Repeat until you achieve desired results.