hi one of two things....
Try setting maxJsonLength property on your web.config:
<configuration>
<system.web.extensions>
<scripting>
<webservices>
<jsonserialization maxjsonlength="Large number here" />
</webservices>
</scripting>
</system.web.extensions>
</configuration>
or because it is WCF you may want to look at the service behaviours. Try setting the maxitemsinobjectgraph property.
<behaviors>
<servicebehaviors>
<behavior name="metadataBehavior">
<servicemetadata httpgetenabled="true" httpgeturl="" />
<servicedebug includeexceptiondetailinfaults="false" />
<datacontractserializer maxitemsinobjectgraph="2147483646" />
</behavior>
</servicebehaviors>
</behaviors>
you may find other examples from here :
http://geekswithblogs.net/mnf/archive/2013/06/02/response-for-rest-method-was-truncated-because-default-maxitemsinobjectgraph-was.aspx[
^]