<!DOCTYPE html>
<body>
  <canvas id="canvas" width="400" height="800"></canvas>
</body>
<script>
  var canvas = document.getElementById('canvas');
  canvas.style.width = (canvas.width / 2) + 'px';
  canvas.style.height = (canvas.height / 2) + 'px';
  var ctx = canvas.getContext('2d');
  ctx.scale(2, 2);
  ctx.fillStyle = 'green';
  ctx.fillRect(0, 0, 100, 150);
  ctx.fillStyle = 'red';
  ctx.fillRect(0, 150, 200, 250);
</script>