I've found one way to do this:

use margins to control dialog position so `margin: -335px 0 0 -280px`;

setting the width you must reset the margins `width: 900px; margin: -250px 0 0 -450px;`

to set the height use max-height `max-height: 800px;`
based on 

<http://www.revillwebdesign.com/change-the-height-width-and-position-of-a-modal-in-twitter-bootstrap/>

Is there a better way to do this? Using this or another approach, is there a way to only have the dialog slide down to `top:0px` and stay there until dismissed?

Using the first margin settings I mentioned does this but obviously as soon as the browser is resized it will get pushed out of view.