<configuration>
<system.web>
<httpRuntime maxRequestLength="1048576" />
</system.web>
</configuration>
From here.
For IIS7 and above, you also need to add the lines below:
<system.webServer>
<security>
<requestFiltering>
<requestLimits maxAllowedContentLength="1073741824" />
</requestFiltering>
</security>
</system.webServer>