Windows Azure Queue can have a message content size up to 64KB per message. This updated was included in Version 2011-08-18 and newer. If you are still on old version, then Windows Azure Queue will have 8KB message.
This size 64KB refers to the size of the UTF-8 encoded message. The body of the request contains the message data in the following XML format.
A message must be in a format that can be included in an XML request with UTF-8 encoding. In the following example the highlighted message-content is considered as message content and its size can go up to 64 KB.
So this 64kb actual message content, not the raw message length, or Unicode binary length, or base64 encoded length when sent via http content.