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.

As @Misha Moroshko has already posted himself, this works:

$("#mainTable").css("width", 100);
$("#mainTable").css("height", 200);

There's some advantage to this technique over @Nick Craver's accepted answer - you can also specifiy different units:

$("#mainTable").css("width", "100%");

So @Nick Craver's method might actually be the wrong choice for some users. From the jquery API (http://api.jquery.com/width/):

The difference between .css(width) and .width() is that the latter returns a unit-less pixel value (for example, 400) while the former returns a value with units intact (for example, 400px). The .width() method is recommended when an element's width needs to be used in a mathematical calculation.