How do you change them
This may be more of an operating system problem that will be best answered at a forum specializing in such issues. I advise trying http://www.tomshardware.com/forum/ and select the forum that addresses your own operating system. There you can post your question and get reliable assistance for your situation.
Yeah, without knowing where you want to change the font and size, it's hard to answer that question. Because of all the recent questions about email, if you are talking about font size & color in an email on the webpage, select the PREFERENCES tab at the top and the MAIL tab on the left hand side, then scroll half-way down the page and edit the Composing Messages data to suit your purposes. Don't forget to press Save at the top left to preserve your choices.