If you use relative font sizing (e.g. fronts size in "em" or percentage) then the user can do this in their browser. For example, in IE you would go View --> Text Size -->Largest. However if the site is written using "pt", "px" etc... font sizing then this does not work.
An alternative would be to have multiple stylesheets for each page and have server-side code (ASP, PHP etc...) to determine which one is to be used.