<link rel="stylesheet" type="text/css" href="../css/style.css">

canvas {
position: absolute;
width: '100%';
height: Ti.UI.SIZE;
top: 0;
bottom: 0;
left: 0;
right:0;
background-color: yellow;
}

<script type="text/javascript" src="../scripts/main.js"></script>

var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
ctx.fillStyle = "blue";
ctx.fillRect(0, 0, canvas.width, canvas.height);

ctx.fillStyle = "red";
ctx.fillRect(10, 10, 40, 40);