I've never found a way to change the default.
But ... you can control the font, size, colour, etc. in the note itself. You can do this by either double-clicking on the task and selecting the Notes tab or by Window | Split and choosing the Notes display in the form at the bottom of the page.
Once there: key in your text, select it, click on the button with an "A" and then set the font, size, colour, etc.