IIS 7.x,

?<system.webServer>
 <security>
 <requestFiltering>
 <requestLimits maxAllowedContentLength="2147483648" />
 </requestFiltering>
 </security>
</system.webServer>

IIS 6.0

?<system.web>
 <httpRuntime maxRequestLength="2097151" />
</system.web>