This exception is thrown because incoming request is greater than allowed maximum request length. By default Kentico CMS 4.0 allows to receive requests that are not greater than 10 MB. This value can be increased by specifying maxRequestLength parameter of <httpRuntime> section of the configuration file (web.config).
1. Open the web.config file in some editor.
2. Add the httpRuntime element in the system.web section as follows:
<httpRuntime executionTimeout="90" maxRequestLength="10000"/>
More detailed information about mentioned parameters can be found in Microsoft documentation.