HTML Canvas Full Screen

I'm playing with the following application using the HTML Canvas: http://driz.co.uk/particles/

At the moment it is set to 640x480 pixels, but I would like to make it full screen as it will be shown a projector. However as far as I can tell I cannot set the canvas size to be 100% as the variables only except numbers and not the %. Using CSS just stretches it rather than making it actual full screen.

Any ideas?

EDIT: Tried finding the height and width using jQuery but it breaks the canvas any ideas why?

var $j = jQuery.noConflict();


var canvas;
var ctx;
var canvasDiv;
var outerDiv;

var canvasW = $j('body').width();
var canvasH = $j('body').height();

//var canvasW     = 640;
//var canvasH     = 480;

var numMovers   = 550;
var movers      = [];
var friction    = .96;
var radCirc     = Math.PI * 2;

var mouseX, mouseY, mouseVX, mouseVY, prevMouseX = 0, prevMouseY = 0;   
var isMouseDown = true;



function init()
{
    canvas = document.getElementById("mainCanvas");

    if( canvas.getContext )
    {
        setup();
        setInterval( run , 33 );
    }
}

function setup()
{
    outerDiv = document.getElementById("outer");
    canvasDiv = document.getElementById("canvasContainer");
    ctx = canvas.getContext("2d");

    var i = numMovers;
    while( i-- )
    {
        var m = new Mover();
        m.x  = canvasW * .5;
        m.y  = canvasH * .5;
        m.vX = Math.cos(i) * Math.random() * 25;
        m.vY = Math.sin(i) * Math.random() * 25;
        m.size = 2;
        movers[i] = m;
    }

    document.onmousedown = onDocMouseDown;
    document.onmouseup   = onDocMouseUp;
    document.onmousemove = onDocMouseMove;
}

function run()
{
    ctx.globalCompositeOperation = "source-over";
    ctx.fillStyle = "rgba(8,8,12,.65)";
    ctx.fillRect( 0 , 0 , canvasW , canvasH );
    ctx.globalCompositeOperation = "lighter";

    mouseVX    = mouseX - prevMouseX;
    mous