How to set width and height dynamically using jQuery

You can do this:

$(function() {
  $("#mainTable").width(100).height(200);
});

This has 2 changes, it now uses .width() and .height(), as well as runs the code on the document.ready event.

Without the $(function() { }) wrapper (or $(document).ready(function() { }) if you prefer), your element may not be present yet, so $("#mainTable") just wouldn’t find anything to…well, do stuff to. You can see a working example here.