Unless you show whats in the mail script its not possible to provide any help.
Even if you do theres no one around here that uses asp that l know of, which is what your mailing script is written in but it might be possible to see how you are getting the 'shirtsize' from the script or maybe not getting it from the script.
According to GoDaddy's website,
Using Windows Hosting Form Mailers | Web & Classic Hosting - GoDaddy Help US
you can't edit the script. So it appears to be a generic contact form script where the form would look something like this and your success page is thankyou.html.
<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Contact Form</title>
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1">
</head>
<body>
<h1>Contact Form</h1>
<form action="gdform.asp" method="post">
<input type="hidden" name="subject" value="Form Submission" />
<input type="hidden" name="redirect" value="thankyou.html" />
<p>Name:
<input type="text" name="Name" placeholder="First & Last" required/>
<p>E-Mail:
<input type="text" name="email" placeholder="you@domain.com" required />
<p>Comments:
<textarea name="comments" cols="40" rows="10">
Type comments here.</textarea>
</p>
<input type="submit" name="submit" value="SUBMIT"/>
</form>
</body>
</html>