When viewing this Codepen Demo on a device on portrait, it works as expected. When veiwing with device orientation lock off and turning to landscape, it fills half the screen.
CODE
<html>
<head>
<script src="https://aframe.io/releases/0.2.0/aframe.min.js"></script>
</head>
<body>
<a-scene>
<a-sky src="https://ucarecdn.com/e1c757bc-73ee-4efe-b068-4f778ce212a3/" rotation="0 -20 0"></a-sky>
</a-scene>
</body>
</html>
I've tried this meta tag with no success:
<meta http-equiv="ScreenOrientation" content="autoRotate:disabled">
HERE"S MY PEN
Have you tried with the latest version of a-frame by chance?
<script src="https://aframe.io/releases/0.8.0/aframe.min.js"></script>
A quick test appears to work on my phone in both landscape and portrait: