If this message is sent by a server to a client it can be hooked. If it isn't sent by a server but generated on the client based on some condition you can't hook it and you have to find that condition.
In general message size shows message size in bytes.
So for example Damage message consists of byte, byte, long, coord, coord, coord. Since long consists of 4 bytes and coord of 2 we will get: 1 + 1 + 4 + 2 + 2 + 2 == 12
Message size == -1 mean that size of this message wasn't specified on its registration. This usually mean that size of a message isn't fixed.