There is a default font size slider right in Preferences.You can enlarge the font for each note with the menu item Format>Font>Bigger (or with the keyboard shortcut Command-+ which is the Command key, the shift key and the plus sign key). Alternately, you can pick a font with Format>Font>Show Fonts (or Command-T).
Making the change the default for all new notes is a good bit more involved and I haven't even tested the method myself.
Thank you! I didn't know that I could change the font. Can this be done in iOS also? When I start typing an email in the mail app then copy and paste from notes the font is always different. Looks bad to me. Thanks again for the preferences tip.There is a default font size slider right in Preferences.