This post addresses the following rules:
Companies may own and operate a realistic amount of non-combat starships where maximum length per ship = ( Tier Number x 1000 ) / 2
Companies may own and operate a realistic amount of combat starships where maximum length per ship = ( Tier Number x 1000 ) / 3 and maximum fleet length = ((Tier Number x 1000) x (Tier Number)) / 2
Only Major Factions may own multiple Star Destroyers. (1000-2000m)
Only Major Factions may own multiple Dedicated Carriers. (1000-2000m)

***​
If I've goofed the math at any point, let me know.
Tier: 1
Maximum Length Per Non-Combat Ship: 500m
Maximum Length Per Combat Ship: 333m
Maximum Combat Fleet Length: 500m
Tier: 2
Maximum Length Per Non-Combat Ship: 1000m
Maximum Length Per Combat Ship: 667m
Maximum Combat Fleet Length: 2000m
Tier: 3
Maximum Length Per Non-Combat Ship: 1500m
Maximum Length Per Combat Ship: 1000m
Maximum Combat Fleet Length: 4500m
Tier: 4
Maximum Length Per Non-Combat Ship: 2000m
Maximum Length Per Combat Ship: 1333m*
Maximum Combat Fleet Length: 8000m
Tier: 5
Maximum Length Per Non-Combat Ship: 2500m
Maximum Length Per Combat Ship: 1667m*
Maximum Combat Fleet Length: 12500m
Tier: 6
Maximum Length Per Non-Combat Ship: 3000m
Maximum Length Per Combat Ship: 2000m*
Maximum Combat Fleet Length: 18000m
*Note that only Major Factions can own multiple Star Destroyers and Dedicated Carriers (each of which is 1000-2000m). Therefore, for example, a Tier 6 corporation can own and operate an 18000m fleet of combat ships, two of which are 1000m or higher: one Star Destroyer and one Dedicated Carrier (both of which have to abide by that Tier's maximum combat ship length). The same goes for Tiers 3, 4, and 5, though Tier 3 can only have a Star Destroyer and a Dedicated Carrier if they're exactly 1000m. Any shorter and they won't be submittable under those ship templates; any longer and they won't be legal for a Tier 3 corporation.
If your eyes are glazing over, here are some examples of how this might work out:
DERP ASTRONAUTICS: Tier 3
FLEET MANIFEST: Doorknob-class Star Destroyer (exactly 1000m); Twit-class Carrier (exactly 1000m); assorted cruisers, frigates, and corvettes (up to 2500m)
BOBCORP: Tier 4
FLEET MANIFEST: Joe-class Star Destroyer (1300m); Bill-class Carrier (1250m); assorted cruisers, frigates, and corvettes (up to 5450m).
SUE INTERSTELLAR: Tier 6
FLEET MANIFEST: Jane-class Star Destroyer (maxed out: 2000m); Jill-class Command Carrier (maxed out: 2000m); assorted cruisers, frigates, and corvettes (up to 14000m)
This is not me speaking as a Factory Judge, just me reading the rules as I see them and trying to take some of the mystery out of corporate fleets.