I would say stick with the size of the weapon. In modern terms one speaks of 125 mm smoothbore Rhinemetal guns of a specific calibre...admittedly the British rated them by the weight of the shot they fired but most other people used the size-calibre. A 6 MW laser would be rather small potatoes, 6 GJ pulse laser is far more reasonable and the missile warheads likely aren't in megatons but 100 kiloton per point. The missile tech speaks of fission warheads anyway which don't have megaton yields (possibly even 10s of kilotons would make sense). Rail and Guass weapons are still more likely to be rated by size, mass of shot or size and acceleration rating. Of all these something like 10 cm Guass launcher and 12.5 cm spinal mount railgun sounds more reasonable. Also you would use joules not watts as what matters is the energy transfer to the target in terms of damage. So a 6 GJ 10 ns pulse 500 Hz, 20 cm focal array equivelent free electron laser, the power requirements of which are likely to be 600 MW.
Our lab laser is a 50 mJ, 10 ns pulse 10 Hz YAG laser. This is considered a class 4 laser (extreme care required).
Damage should remain damage just in points because its probably best to keep it abstracted as you need to see what sort of armour you might want to have (which is in points) and it makes it easier to compare weapons.
