ASP.net Background size in pixels

Hi

I am creating a gradient background in Photoshop for my ASP.net screen background.
What width and height in pixels should I use for the standard ASP.net background?
The image I am creating will be specified as Green1 in Site.css


/* DEFAULTS
----------------------------------------------------------*/

body  
{
    background: #001E00;
    background-image: url('/Resources/Green1.jpg');
    font-size: .80em;
    font-family: "Helvetica Neue", "Lucida Grande", "Segoe UI", Arial, Helvetica, Verdana, sans-serif;
    margin: 0px;
    padding: 0px;
    color: #696969;