My preffered method is to set a cookie and reload the page. Every page load checks for the cookie and if nessecary adds a reference to another stylesheet.
If you just want to change the font size you could add style="font-size:150%;" or similar to the <body>.
Loading a completely seperate stylehseet allows you to do much more than increase the font-size.
i use php for this.
Ben
[added] ^^ what le_gber said ^^ [/added]
