How can I change the font and font size for the entire website ?

QUICK: Hold CTRL and roll your scroll wheel.

Real answer, edit your themes style.css to include something like

... whether that works depends on your chosen theme design - whether it used % or em when setting font sizes, or whether it's hard-coded pixel.point sizes (which you cannot override globally)

Most of the time this will give very mixed results, with some things (hopefully the content text) scaling as requested) and other bits (like graphical nav elements) breaking the layout quite badly at extreme scales. Using the scrollwheel trick will display how much work is required to make it work.

It's a really big job unless you actually understand CSS!

.dan.
How to troubleshoot Drupal | http://www.coders.co.nz/