How to get MSMQ message queue hard disk usage?

MessageQueue class has a GetAllMessages() which returns an array of Message instances.
Once you get this array, initialize a counter variable to zero. Then loop through all messages in the array, and use the BodyStream property of the Message instance to get a reference to the underlying stream. Finally, use the Length property of the stream to get its length (in bytes), and add it to the counter.

Message[] messages = queue.GetAllMessages();
long total = 0;
foreach (Message message in messages)
{
   total += message?.BodyStream?.Length;
}