Though I reccomend against doing it this way(more on that at the end), here is what you do to change the base font size with css; type in the box:
body {font-size:16px}
And then press update at the bottom, you can make that 16 bigger or smaller as you desire (for reference its defualt is 12px), its important to keep the px after the number.
This wont update every font size though, so it may make the page harder to read, in the sense that the headings may be smaller than the text.
Why I reccomend against this, is that this may break the layout a little, the correct thing to do is to use your web browsers font resize function.
This wont work properly if you use internet explorer, if you have ie7 you need to use the 'Zoom' function instead.
All other web browsers will resize the text with the normal text size settings, which is one of the main reasons to reccomend Firefox, Safari or Opera.
If your uses are stuck with ie6 then the css is the only way to go for the moment, though its an area we hope to improve on later.
Hope this helps.