As you follow @Alan-Kilborn’s advice to,

Under the Settings menu is something called Style Configurator, where all fonts are set. Play around.

I wanted to let you know: when people start playing around wanting to change the font size, they often first reach the Global Styles in the Language: column, and Global override in the Styles: column – this is not the one you want to change. You want to go to Default style in the Styles: column instead.

See the User Manual’s section on Settings > Style Configurator > Global Styles for more.

To control the size of the letters in the files you are editing you adjust the zoom level. The quick way is hold the Ctrl key down and to roll the mouse wheel up/down. Another way is to hold the Ctrl key down and to use the + and - keys on the numeric keypad. With this you also have Ctrl+/ using the / on the numeric keypad available to reset the zoom back to the default. You can also use the View / Zoom / ... menu options.

There is a Windows setting under Settings / System / Display / Scale & Layout (section) / Scale that will let you adjust the size of text throughout Windows and most applications, including Notepad++.

You may want to experiment with Notepad++'s dark mode.

The thing @Alan-Kilborn mentioned will give you a more fine grained control than any of the things I listed above.

Under the Settings menu is something called Style Configurator, where all fonts are set. Play around.