RadControls for ASP.NET AJAX 
Problem
When you load a lot of nodes via WebService you receive an
error "Maximum length exceeded".
Solution
Extend the maxJsonLength property, by adding the following
section to your web.config file, at the configuration level, after the
system.web node:
CopyXML
<system.web.extensions>
<scripting>
<webServices>
<jsonSerialization maxJsonLength="5000000" />
</webServices>
</scripting>
</system.web.extensions>