<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
</head>
<body>
<input id="hiddenX">
<input id="hiddenY">
<script>
document.getElementById('hiddenX').value = document.body.clientWidth+1;
document.getElementById('hiddenY').value = document.body.clientHeight+1;
</script>
<!-- остальной код разметки -->
</body>
</html>