There is an annoying bug on Evernote for Windows (I have not tested this in other platforms) that has been around for over a year now I believe.
If I try to set my default font size to 10 (under options->note font), Evernote will keep reverting to size 9. Oddly enough, if I set it to 11, or 12, or apparently any other, this issue does not occur.
Please, someone have a look at this and fix it, as font size 9 is too small for me, and 11 is a bit too big, so I've been living with the annoyance of manually changing the font size to 10 for a long time now.