ASP.NET HttpException: Maximum request length exceeded

Limite por defecto para uploads en Web.Config es 4096 (4 megabytes)

Las siguientes lineas cambian a 20MB

<configuration>
...
<system.web>
...
<httpRuntime maxRequestLength="20480" />
</system.web>
</configuration>


Otras Configuraciones

<httpRuntime executionTimeout="300" maxRequestLength="51200"/>
<httpRuntime
executionTimeout="1200"
maxRequestLength="102400"
useFullyQualifiedRedirectUrl="false"
minFreeThreads="8"
minLocalRequestFreeThreads="4"
appRequestQueueLimit="100" />

Nota:
maxRequestLength<=1048576 (1 GB) para .NET Framework 1.0/1.1 y
maxRequestLength<=2097151 (2 GB) for .NET Framework 2.0.

Referencias
FileUpEE - Large Uploads in ASP.NET:
http://support.softartisans.com/docs/fileupeev4/doc/dotnet/aspdotnet_largeuploads.asp

FileUpSE - large uploads in ASP.NET:
http://support.softartisans.com/docs/fileupv4/prog_g_dotnet_largeuploads.htm