if your using 3d then I would say four bytes for X, Y, Z each as blitz uses floats and it will give better results.
Elite Multiplayer uses:
header x bytes (dependent on version)
X, Y, Z, rotation X, Y, Z 4 each
system 1 byte
galaxy 1 byte
Special action 1 byte
footer x bytes
Elite Multiplayer uses:
header x bytes (dependent on version)
X, Y, Z, rotation X, Y, Z 4 each
system 1 byte
galaxy 1 byte
Special action 1 byte
footer x bytes