I'm not sure I would like that. Having a Lilith fire off something SRed sized that OHKOs cruisers doesn't make much sense.
That, and it's easier to tell exactly what beam is being fired if the sizes remain consistent across ships.
I think the Terslash has quite a bit of room to be bigger. Personally, I always envisioned it to be near the same size of the BGreen, and it seems to me that the Deimos could still mount slightly larger beam/beam glows. The problem now is (judging from the Hecate above), the terslash looks to be about 2x as thick as an AAA, and that's kinda impotent looking.