From 'suggestions for 1.3' comments.

The length encoding was extended for version 1.2 to support packets > 255 octets in length, in a backward compatible way.  For packets of length > 255, the length field is 3 octets rather than optimally 2.

Some options:

1) Always use 2 octets for the length

2)  Length field is 1 or 2 octets long.  0 to 6 in the first octet means there is a second octet. The length can be up to 6*256 + 255 (=1791).

3) the MQTT algorithm, perhaps restricted to fewer octets.