document.getElementsByTagName('body').style.fontSize = '110%';
This would increase the sizes of all of the fonts defined within the web page to be 10% bigger than they were previously (assuming that you start with the font size on the body set to 100% and have all the font sizes in the page defined using em and not a totally inappropriate and non-scalable measure such as px).
To change the font size for just one section of the page having a specific id (eg. thissec) you would use the following:
document.getElementById('thissec').style.fontSize = '1.1em';
Both of those examples make the targetted content 10% bigger. If you want to scale it by a different amount then you would of course assign a different value.
This article written by Stephen Chapman, Felgall Pty Ltd.