Changing body tag style through JavaScript

I'm trying to write a script to change the width of the page, considering user's client width. It's something like this:

function adjustWidth() { 
    width = 0;
    if (window.innerHeight) {
        width = window.innerWidth;
    } else if (document.documentElement && document.documentElement.clientHeight) {
        width = document.documentElement.clientWidth;
    } else if (document.body) {
        width = document.body.clientWidth;
    }
    if (width < 1152) {
        document.getElementsByTagName("body").style.width="950px";
    }
    if (width >= 1152) {
        document.getElementsByTagName("body").style.width="1075px";
    }
}
window.onresize = function() {
    adjustWidth();
};
window.onload = function() {
    adjustWidth();
};

With this script I get an error from Firebug:

document.getElementsByTagName("body").style is undefined

Now my question is, how can i access the style of body? Because in the css sheet its selector and width property are defined.