Maximum request filesize is globally controlled through the application web.config httpRuntime element.
The maxRequestLength attribute specifies the limit for the input stream buffering threshold, in KB. This limit can be used to prevent denial of service attacks that are caused, for example, by users posting large files to the server.
To enable large file uploads, you need to change the value of this attribute to the largest allowed combined file size for your application. If someone selects and uploads files, with total size larger than maxRequestLength, s/he will get the "Page not found" error (which is the default error of the Framework).
If you use IIS 7 you should also modify requestLimits Element for requestFiltering (IIS Settings Schema) and maxAllowedContentLength attribute.
All the best,