CSS Body background graphics

Hi

I have recently been trying to redesign  a webpage that incorparates images from photo shop and illustator. When I click new on

photoshop or illustator to create the background image I am not sure what the width and height in pixels the

image should be. So that when I incorparate the image  onto the  web page body its just the right size i.e it does not repeat or is not  too short

or too big when viewed on different screens. Can any one help.

This is the css that displays the image on web. What would be the right width and height in pixel to design my background image using photoshop or illustrator.

body {
background: url(images/img03.jpg);
margin: 0px;
padding: 0;
font-family: Arial, Helvetica, sans-serif;
font-size: 12px;

}