The File Upload server control provided by ASP.NET allows users to upload one or more files to the server. By default, ASP.NET permits only files that are 4,096 KB (or 4 MB) or less to be uploaded to the Web server. To upload larger files, you must change the maxRequestLength parameter of the <httpRuntime> section in the Web.config file.
<httpRuntime executionTimeout="240" maxRequestLength="20480" />
The value of maxrequestlength is in KB. In this case it's allowed up to 20 MB