If you ever find the following error in Reporting Services after uploading a file or creating a new subscription:
“Maximum request length exceeded"
There is a simple reason for that:
The default value of the property MaxRequestLength is 4 Mb. When you upload a report or create a subscription, if the report size is higher than this value, you will bump into this error.
In order to increase this value, you can modify this property under the httpruntime element in the file Web.config of the Report Manager (%\Program Files\Microsoft SQL Server\MSSQL.X\Reporting Services\ReportManager) and in the Report Server (%\Program Files\Microsoft SQL Server\MSSQL.X\Reporting Services\ReportServer).
Then, you will have to restart the RS service before trying again.
By default, the property MaxRequestLength doesn´t exist in the config files. To modify it you would have to add it to the section. For example:
<httpRuntime ... maxRequestLength="10240" .../>
To define the right value, you could export the file and check its size. More information in the following link:
Report Size and Limits
Reporting Services Support Engineer