Вроде один пиксел:
<!DOCTYPE html>
<html>
<head></head>
<body>
<canvas style="width: 100px; height: 100px;"></canvas>
<script style="text/javascript">
var el = document.getElementsByTagName('canvas')[0],
context = el.getContext("2d");
context.moveTo(0,0);
context.lineTo(100,100);
context.strokeStyle = '#000';
context.lineWidth = 1;
context.stroke();
</script>
</body>
</html>