here is a very basic example as to how i would approach something like this using ASP.
Code:
<%
dim intTextSize
dim varTextQuery
varTextQuery = Request.Querystring("text")
If varTextQuery = "large" then
intTextSize = "4"
Else
intTextSize = "2"
End If
%>
<body>
<p><h="<% = intTextSize %>">Sample of text content on page</h></p>
<p>
<%
If varTextQuery = "large" then
%>
<a href="example.asp">Small Text</a>
<%
Else
$>
<a href="example.asp?text=large">Large Text</a>
<%
End If
%>
</p>
</body>
in the case that you are trying to change the text size of multiple areas of the site, i would suggest calling two predefined css files in a similar fashion, instead of hard coding it into the page as i did above.
hope that helps a little.