web service via javascript problem

Check your web.config file. You will see code like below. The maxJsonLength is set to 500. Which is why you are getting the error.

<!--

Uncomment this line to customize maxJsonLength and add a custom converter-->

<!--

<jsonSerialization maxJsonLength="500">

<converters>

<add name="ConvertMe" type="Acme.SubAcme.ConvertMeTypeConverter"/>

</converters>

</jsonSerialization>

-->