increase font size

Here’s a good resource for learning some CSS:

http://www.w3schools.com/css/

Firebug is a great tool for working with CSS — it will help you identify which CSS code you need to change.

Without a link to your site, no way to offer any more specific help.

In order to change anything, you’ll need to have a CSS custom option in your theme or use a custom CSS plug-in or create a Child Theme.