setcookie has an expire parameter, which you can use to set desired expiration date. docs have an example on how to do this.
just create another cookie with font size information and invoke relevant stylesheet. radio button need to be inside the form tag to be send as a part of http request.
When it comes to the font resizing I would personally say, you are better off changing the size on the client side, with jQuery for example. I think generally if someone requires a larger font size on every page (for accessibility) they would have the font size increased in the browser.
Although you could also do an AJAX call to set a cookie / save a record in the DB if you wanted to change stylesheet elements / set a higher font size permanently.
You'd be better off using $_SESSION to persist the style across pages. That way it works even if the user has cookies turned off, because it'll rewrite URLs with the session ID for you.