How can I disable comment and contact textarea resizing?

Why do you need fixed-size text areas? I can't imagine a reason...

So ssb, Basically you have to modify the form before it is built you can do this in a module quite simply and it would be a good starter module for you if you haven't made one before. But here is a quick and dirty way:

Create a page and use it as the contact page, copy and paste the code below into the body and make sure the input filter is set to php.

  //give your function a unique name:

  function local_contact_form_page(){

    //first we get the contact form to manipulate:
    $form = contact_mail_page();

    //then we can start to alter default settings:
    $form['message']['#resizable'] = false;

    //you could also set the rows and cols:
    $form['message']['#rows'] = 20;
    $form['message']['#cols'] = 40;

    //and return the form:
    return($from);
  }

  // then we need to know what to do with the form on submit:
  function local_contact_form_page_submit($form_id, $form_values){
    
    //send the parameters to the contact form submit:
    return(contact_mail_page_submit($form_id, $form_values));
  }

  //and finally we just need to output the form to the user:
  print drupal_get_form('local_contact_form_page');

And that should do it. You can use this in blocks too and you can do more with the contact form too.