The default content-box font is 12px, Arial,Helvetica,sans-serif. This can be changed however via custom CSS code added to the Custom HEAD area of the System Settings page.
Here's what you'd add to the Custom HEAD Code section on the System-wide Settings page:
<style>
.innerbox {font: normal 12px Arial, Helvetica, sans-serif;}
#sidelist li {font: 12px Arial, Helvetica, sans-serif;}
.linklist li {font: 12px Arial, Helvetica, sans-serif;}
</style>
Just replace the 12px with whatever font size you'd like.