I would say the size of game would only be limited by the amount of space to play in and your ability to build large fleets of Nefs/Digs.
I assume the scale chosen for the rules was a compromise between the amount of ships you can comfortably fit onto a 4x8 ft table and having reasonable detail on the models.
I'd love to see a game played at maybe 1/300 or even 10mm, using scratchbuilt ships with land forces etc from commercially available figures.
Your 1/600 idea is certainly feasible, but would involve a fair amount of work building the fleets.
I hope it "takes off".
Sorry about that. I'll get my coat....