I think the reason you get the error is because the MSMQ adapter might be giving you a non-seekable stream, which doesn't have a way to query the length of the stream. That's actually quite normal and fairly common when working with streams. Why do you need the length of the message to decrypt it?
(Mind you each adapter will return streams in messages with particular characteristics; that's something you'll encounter pretty often in BizTalk and it's something to keep in mind)