How can the font size be changed in Notepad++?

Ctrl and NUMPAD +/NUMPAD - or mouse wheel to zoom in/out

Edit: Or to achieve exactly what you asked for...

Settings > Style Configurator. Under Font Style you can set the font and size

Open menu Settings/Style Configurator. Then with the default theme "Default (stylers.xml)" and default language "Global Styles", change the font name and font size for style "Default Style" (the default, style "Global override" will not cut it).

If a document is already open then the display will change immediately as the font name and font size is changed - nice touch! Press the "Save & Close" button to keep the new settings.

Note that in Notepad++ 6.0, you will also need to check the enable global font and enable global font size check boxes.

Alternatively, hold Ctrl and use the scroll wheel of your mouse.