Q:

The length of the string exceeds the value set on the maxJsonLength property (WebMethod)

Background
I have a WebMethod in the code-behind for an ASPX page:
[WebMethod]
public static string GetServiceDetail()
{
    //...
}

I'm using jQuery to send an AJAX request to it:
$.ajax({
    type: "POST",
    url: url + "GetServiceDetail",
    data: JSON.stringify({}),
    contentType: "application/json; charset=utf-8",
    dataType: "json",
})

This generally works correctly. 
Problem
When the WebMethod returns about 88KB of data, the framework returns a 500 Internal Server Error to the HTTP request.   The HTTP response body contains a bunch of .NET exception information in JSON format.  When deserialised, the response has this information:

System. InvalidOperationException:
Error during serialization or deserialization using the JSON JavaScriptSerializer.  The length of the string exceeds the value set on the maxJsonLength property. 

   at System. Web. Script. Serialization. JavaScriptSerializer. Serialize(Object obj, StringBuilder output, SerializationFormat serializationFormat)
   at System. Web. Script. Serialization. JavaScriptSerializer. Serialize(Object obj, SerializationFormat serializationFormat)
   at System. Web. Script. Services. RestHandler. InvokeMethod(HttpContext context, WebServiceMethodData methodData, IDictionary`2 rawParams)
   at System. Web. Script. Services. RestHandler. ExecuteWebServiceCall(HttpContext context, WebServiceMethodData methodData)

Troubleshooting

I'm not using JavaScriptSerializer in the WebMethod at all.  As the stack trace shows, the exception is happening outside of my code. 
Existing StackOverflow questions seem to be about explicit JSON serialisation or different technologies.  They imply changing the Web.config value doesn't work. 

A:

I solved this by changing an attribute called maxJsonlength in Web.config:
<configuration>
  <system.web.extensions>
    <scripting>
      <webServices>
        <jsonSerialization maxJsonLength="4000000"/>
      </webServices>
    </scripting>
  </system.web.extensions>
</configuration>

Sourced from https://stackoverflow.com/a/1151993/238753.