I solved my problem and in case any body has this problem in future, here is the solution.
My actual problem was not the length of the JSON object. It was the numbers of keys in that object. Actually there is a limit for the keys number you can send via the ajax post request in ASP.NET and in case you need more, youe should increase it in web.config appSettings section:
<add key="aspnet:MaxHttpCollectionKeys" value="100000" />
And also don't forget to add these two configs in similar cases to some big numbers:
<appSettings>
<add key="aspnet:MaxJsonDeserializerMembers" value="10000000" />
</appSettings>
and
<system.web.extensions>
<scripting>
<webServices>
<jsonSerialization maxJsonLength="200000000"/>
</webServices>
</scripting>
</system.web.extensions>