I faced this problem recently. There is a property called maxRequestLength which needs to be increased in the machine and web.config file which is in the following location.

C:\Program Files\Microsoft SQL Server\MSRS10.MSSQLSERVER\Reporting Services\ReportServer

Late to the party, but I've seen this happen with reports that have multiple embedded images that you may have deleted from the layout but are still there under Report Data -> Images. They can balloon the size of the report, and it's easy to check before changing the max request length settings in IIS.

"SQL Reporting Services Error- Maximum request length exceeded"

This can be resolved by increasing the connection timeout property in web.config file in reporting server.
You’ve got to adjust 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

<httpRuntime executionTimeout="9000" requestValidationMode="2.0" />

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"

Ref: https://www.isolutionspartners.com/2011/09/16/sql-reporting-services-error-maximum-request-length-exceeded/