I had the problem of opening a modal popup in my ASP.NET application and showing more information when the user clicks a checkbox or radio button and the data was too big for the modal or the dropshadow wouldn’t match up with the parent window.  I searched all over the web and posted a question on StackOverflow but I couldn’t find anything.

Finally I got the brilliant idea to check the source code to see what happens  in there and then I found the answer:

$find(‘ModalPopupExtenderClientID’)._layout();

Just add that javascript to the event where you are changing the content of the modal and it will resize.