How big is an MSMQ message in the outgoing queue?

There's always something you didn't know you didn't know.

For example, learning about the extra columns in Computer Management for MSMQ.

From the View menu, select "Add/Remove Columns..." and move "Message Quota Used" to the right hand side.

Bingo - you can now see total message sizes (as opposed to just that of the message body) in outgoing queues.

In the screenshot below you can see I have sent 10 non-transactional and 10 transactional messages. Each message has a payload of 100,000 characters - which take up 200,000 bytes as they are stored as double-byte Unicode.

The non-transactional messages have a message header of 252 bytes each while the transactional messages have a greated overhead of 276 bytes to accommodate the extra sequencing data.

Sadly this information is not available on the destination queues, only the outgoing queues.