I have to make an A4 newsletter for a project and I was wondering if anyone knew the average font size of A4 newsletters?

That’s a really funny question. There is no real standard for newsletters – you should just fire up your software, write the newsletter, and then think about font size and clarity later.

You should definitely use a nice serif font for clarity, but the rest is for you to judge. Font size, and printed size, aren’t so closely related anyway – so judge from the print preview.