(413) Request Entity Too Large | uploadReadAheadSize

That is not problem of IIS but the problem of WCF. WCF by default limits messages to 65KB to avoid denial of service attack with large messages. Also if you don't use MTOM it sends byte[] to base64 encoded string (33% increase in size) => 48KB * 1,33 = 64KB

To solve this issue you must reconfigure your service to accept larger messages. This issue previously fired 400 Bad Request error but in newer version WCF started to use 413 which is correct status code for this type of error.

You need to set maxReceivedMessageSize in your binding. You can also need to set readerQuotas.

<system.serviceModel>
  <bindings>
    <basicHttpBinding>
      <binding maxReceivedMessageSize="10485760">
        <readerQuotas ... />
      </binding>
    </basicHttpBinding>
  </bindings>  
</system.serviceModel>

I was having the same issue with IIS 7.5 with a WCF REST Service. Trying to upload via POST any file above 65k and it would return Error 413 "Request Entity too large".

The first thing you need to understand is what kind of binding you've configured in the web.config. Here's a great article...

BasicHttpBinding vs WsHttpBinding vs WebHttpBinding

If you have a REST service then you need to configure it as "webHttpBinding". Here's the fix:

<system.serviceModel>

<bindings>
   <webHttpBinding>
    <binding 
      maxBufferPoolSize="2147483647" 
      maxReceivedMessageSize="2147483647" 
      maxBufferSize="2147483647" transferMode="Streamed">
    </binding>  
   </webHttpBinding>
</bindings>