kuzya_vl,
Вот рабочий код
<html>
<head><script>
function getCellSize()
{
alert(document.getElementById("mycell").offsetWidth);
alert(document.getElementById("mycell").offsetHeight);
}
</script>
</head>
<body onload=getCellSize();>
<table>
<tr>
<td>олоол</td>
<td id="mycell">ололо</td>
<tr>
</table>
<body>
</html>