From here:

You could, quite honestly, run into this error in a lot of situations involving a web app, but we’re talking specifically about SSRS in this post. The basic problem here, is that your posting an amount of data to a web app larger than it is configured to accept.  Hence, it is throwing an error.

It’s an easy fix though.  You’ve got to adjust the web.config for the web app, which in the case of reporting server, is usually somewhere like this:

For SQL Server 2012: C:\Program Files\Microsoft SQL Server\MSRS11.MSSQLSERVER2012\Reporting Services\ReportServer

Find the web.config file for your reporting services instance, open it up, and track down the line that looks something like this

executionTimeout = "9000" />

Now just add a max request length attribute in there to fix the problem, adjust your size as needed.  This is 5meg.

executionTimeout = "9000" maxRequestLength="500000" />

And now you’ll need to restart IIS.  start->run->”iisreset”