What do I add to this ONSUBMIT script so it makes the popup window allow for SCROLL BARS if it's too high?

<FORM METHOD="POST" ACTION="http://www.camarosource.ca/list_the_rpos.php" name="form_rpo" onSubmit="postToPopup(500,335,this)">

I have a RPO code decoder. which when you click a Forum Submit button, a new window pops up and displays all the decoded RPO Codes into a new popup window. The WIDTH of the window is 335pixels, however the HEIGHT is currently set at 500pixels. The problem is that the page can EASILY and most of the time is, LARGER than this height. As a result the information below 500pixels is hidden. I want to be able to pop up scroll bars IF it's larger than 500pixels.

Someone posted that script above to me. But I never could figure out what the "postToPopup(500,335,this)"> statement, the "this" is for?? Obviously "500" is for a set height, 335 for set width.. but "this"?

And of course HOW to make the height NOT set so it allows scroll bars. Thanks