Error 2 There was an exception running the extensions specified in the config file. ---> Maximum request length exceeded.
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, and simply saying "no!"
It's an easy fix though! You've got to tweak the web.config for the web app, which in the case of reporting server, is usually somewhere like this:
C:\Program Files\Microsoft SQL Server\MSSQL.2\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
Now just add a max request length attribute in there to fix the problem, adjust your size as needed. This is 5meg.
And now you'll need to restart IIS. start->run->"iisreset"