How is the property in webconfig maxRequestLength measured

maxRequestLength is measured in kilobytes

maxAllowedContentLength is measured in bytes

It accepts KB. For 50 MB, set it to

maxRequestLength="51200"

The property maxRequestLength indicates the maximum file upload size supported by ASP.NET. This limit can be used to prevent denial of service attacks caused by users posting large files to the server. The size specified is in kilobytes. The default is 4096 KB (4 MB). MSDN

For 50 MB you will need to set it to 51200.

<httpRuntime maxRequestLength="51200" executionTimeout="300"/>

Edit based on comments

The OP does not ask about executionTimeout but @barnes did in comments below. I feel to add some details about executionTimeout as well which is other httpRuntime attribute.

executionTimeout:

Optional TimeSpan attribute. Specifies the maximum number of seconds that a request is allowed to execute > before being automatically shut down by ASP.NET. This time-out applies only if the debug attribute in the compilation element is False. To help to prevent shutting down the application while you are debugging, do not set this time-out to a large value. The default is "00:01:50" (110 seconds), MSDN.