help checking the length of a field in an HTML/ASP form

I know NOTHING about ASP, but I've been able to search and piece together a form I can use on a site to have a visitor submit their name, email address, and an "activation code" and have the data sent to me via email (along with their IP address).

The last thing I'd like to do, however, to try and minimize spam is check to see if the "activation code" field is a certain length.  If the field is too long or too shortl, I don't want the form to submit.

Here's what I'm using in HTML:

 

<form method="POST" action="contactusprocess.asp">
  <table>
    <tr>
      <td>Name:</td>
      <td><input name="Name" size="50" maxlength="50"></td>
    </tr>
    <tr>
 &nb ...