If you're not tied to the idea of the GET method (that is, creating a link with a query string), you might look to using POST (form submission).

<form method="POST" action="second_script.cgi">
<input type="hidden" name="username" value="whatever">
<input type="hidden" name="show_size" value="some ridiculous number li
+ke 8 3/4">
<input type="hidden" name="belabor the point?" value="Yes & it's fun!"
+>
<input type="submit">
</form>
[download]

The best part about that is that you can use a heredoc or interpolate variables into the value sections. The browser or user agent will be the one to handle escaping the parameters correctly.

Using CGI, there's no need to wonder if users use GET or POST to send data. Your interface is the same.