Change font size for PDF Text comments in Preview
You have to use defaults write in a slightly differently way with Preview and other sandboxed apps.

Using defaults write com.apple.Preview ... modifies (or creates) a plist file in ~/Library/Preferences. 

As pointed out, that's not where Preview saves (or looks for) its preference file, which is in a nested subfolder of ~/Library/Containers instead.

To modify the correct plist, use: defaults write -app Preview ... 

You'll need to make sure you have the correct key, too. I have the following keys in my Preview plist:

"PVAnnotationFontSize_12" = 10;
"PVAnnotationFontSize_17" = 10;
"PVAnnotationFontSize_5" = "11.99199";
"PVAnnotationFontSize_6" = 10;


However, modifying those does not change the text size for either Annotations or Notes. Annotations seem to keep the last settings used. Notes don't seem to be editable in the UI.

Also, note that you don't need to use grep to read a specific plist key. E.g. defaults read com.apple.Preview PVAnnotationFontSize_5

EDIT: Having probed into the plist data and the text strings within the Preview app itself, I'm not convinced that there is an editable key for the font size or style in Preview's Notes.