I remember you asked this last night.
This procedure is a bit techy but it's a good exercise. There are many small tweaks that can be done in some apps using a similar procedure.
This post is a couple of years old
, but almost nothing's changed.
1. Find the file DefaultFonts.plist as follows
Control click or right click on Notes and choose Show Package Contents
Open Contents/Resources/en.lproj and find DefaultFonts.plist (The en is for English. If you want to use another language, find the right folder.)
drag that file to the desktop so you can save an unchanged version.
If you're happy with one of the three default fonts (I'd use Helvetica Neue, myself), you can just change the size listed in the plist file.
3. Double click DefaultFonts.plist. It'll open in TextEdit.
4. Change the size number for the font you like. For example:
5. Save the file. Quit Notes and open it again.
If you want to add a default font choice, you can follow the directions in the post I linked to above.
If all goes well, save the original file you copied in a well named folder. If something's wrong, option drag the original back into en.lproj and let the OS replace the file you changed. Then start over.