How to Solve Method Error 500 - Maximum Length Exceeded with AJAX web service call

At client side I've been assigned a bug concerning
a combo box. When the combo box is changed by the
user, a secondary combo box has its values reloaded.
This is done by calling a web service. The bug is
that "Method Error 500" is displayed as the only
item on occasion. I checked and cross checked the
problem with my own post
http://jatindersingh.blogspot.com/2008/03/method-error-12030.html  ,
but could not sort the error , search on google lead to
the following solution.

Solution:

After some rooting around, it turns out that this is
caused by too much data being returned in the SQL query
behind the web service call, so the web service returns
an error. A simple change to the web.config file can
increase the size limit of data returned:

<system.web.extensions>

  <scripting>

    <webServices>

       <jsonSerialization maxJsonLength="5000000" />

    </webServices>

  </scripting>

</system.web.extensions>

Explaination:

First off, the serialization process will fail if the
number of nested objects is greater than that defined
within the RecursionLimit property , which is 100.
The serialization process will also fail if the length
of the serialized text is greater than that of the
MaxJsonLength property; again, note the default value
of 2,097,152 base ten.

Source http://www.codeproject.com/KB/ajax/ajax_json_serialization.aspx 

Hope visitor find it useful.