Add the following to your web.config:
<system.webServer>
<security>
<requestFiltering>
<requestLimits maxQueryString="32768"/>
</requestFiltering>
</security>
</system.webServer>
See:
http://www.iis.net/ConfigReference/system.webServer/security/requestFiltering/requestLimits
Updated to reflect comments.
requestLimits Element for requestFiltering [IIS Settings Schema]
You may have to add the following in your web.config as well
<system.web>
<httpRuntime maxQueryStringLength="32768" maxUrlLength="65536"/>
</system.web>
See: httpRuntime Element (ASP.NET Settings Schema)
Of course the numbers (32768 and 65536) in the config settings above are just examples. You don’t have to use those exact values.