Add following lines of code in your web.config.
<system.web>
<httpRuntime maxRequestLength="1073741824" targetFramework="4.5" />
</system.web>
if you are running on IIS 7 or later add following lines.
<system.webServer>
<security>
<requestFiltering>
<requestLimits maxAllowedContentLength="1073741824" />
</requestFiltering>
</security>
</system.webServer>