Form validation script for client side cart

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Required field(s) validation

http://www.dynamicdrive.com/dynamici...uiredcheck.htm

I have a client side shopping cart. The problem is with my form. If someone does not choose a size it will still add to cart. I want a window to pop up saying please choose a size if none has been chosen. I have entered some code to try to validate the form but it is not performing correctly. Not sure where or how to use the Required field(s) validation script.
Thanks

<form action   = "javascript: void 0;"
      method   = "post"
	  onsubmit = "SetDesc ('shirt');
                           if(document.getElementById('size').selectedIndex < 1) {
                           window.event ? window.event.returnValue = false :e.preventDefault = true;alert('Please select a size');
  }
                  SetCode ('ts00014');
                  SetImg ('images/cartimage/shirt.gif');
                  SetPcat (2);
                  if (ReadForm (this, true))
                    CallView ();
                  ClearAll ();
                  return false;">

  $17.95 - 
  <input type="hidden" name="initer" value=" @0" />
  <select id="size" size="1" name="size">
 
    <option selected value="Select a size">Select a size</option>
    <option value = "Small @17.95">Small</option>
    <option value = "Medium @17.95">Medium</option>
    <option value = "Large @17.95">Large</option>
	<option value = "X-Large @17.95">XL</option>
	<option value = "2XL @19.95">2X ($2)</option>
  </select>
  <input type    = "submit"
         name    = "submit"
         value   = "add to cart" />

<br /><br />
Input <b>Quantity</b> to order - 
  <input type  = "text"
         name  = "qty"
         value = "1"
         size  = "4" />
<br /><br />
</p>
</form>

[boogyman]

try ?

if(document.getElementById('size').value == "Select a size") {

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I tried but it was unsuccessful.

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Here is the where I got my original code fromhttp://members.aol.com/paypalcart/index1.html