Background width, help! Options

[boabawhales]

CODE

<!-- begin code provided by createblog.com -->
body {
background-image:url(PUT URL HERE IF YOU HAVE A BACKGROUND IMAGE, IF NOT, DELETE EVERYTHING INSIDE THE PARENTHESES);
background-repeat: repeat-y-x;
background-color: #COLOR CODE;
scrollbar-face-color: #COLOR CODE;
scrollbar-shadow-color: #COLOR CODE;
scrollbar-highlight-color: #COLOR CODE;
scrollbar-3dlight-color: #COLOR CODE;
scrollbar-darkshadow-color: #COLOR CODE;
scrollbar-track-color: #COLOR CODE;
scrollbar-arrow-color: #COLOR CODE;
}

<!-- end code provided by createblog.com -->

i want to make the background image to align in the middle and only repeat virtically. and also, i would like it to be 950px wide. what EXACTLY should i add or change?