The ship below IS game breakingly large:
Exodus class Migration Vessel 2,959,707,023 tons 5,938,794 Crew 33,647,068.2 BP TCS 59,194,140 TH 16,000,000 EM 0
270 km/s Armour 100-183629 Shields 0-0 HTK 527491 Sensors 11/11/0/0 DCR 1010 PPV 0
MSP 101,105 Max Repair 2400 MSP
Hangar Deck Capacity 250,000 tons Cargo 1,000,000 Cryogenic Berths 200,000,000 Habitation Capacity 1,000,000,000 Passengers 10000000 Cargo Shuttle Multiplier 10 Tractor Beam
Rear Admiral (Lower Half) Control Rating 4 BRG AUX ENG SCI DIP
Intended Deployment Time: 3 months Flight Crew Berths 5,000
Jump Point Stabilisation: 90 days
Recreational Facilities
Fuel Harvester: 2500 modules producing 100,000,000 litres per annum
Terraformer: 4000 modules producing 1 atm per annum
Orbital Miner: 2500 modules producing 25,000 tons per mineral per annum
Maintenance Modules: 250 module(s) capable of supporting ships of 250,000 tons
Refuelling Hub - Capable of refuelling multiple ships simultaneously
Ordnance Transfer Hub - Capable of transferring ordnance to multiple ships simultaneously
Shepherd-Jordan Commercial Magneto-plasma Drive EP3200.00 (5000) Power 16000000.0 Fuel Use 2.52% Signature 3200.00 Explosion 5%
Fuel Capacity 2,000,000,000 Litres Range 4.8 billion km (207 days at full power)
Shepherd-Jordan CIWS-250 (1000x8) Range 1000 km TS: 25,000 km/s ROF 5
Shepherd-Jordan Active Search Sensor AS31-R50 (50%) (1) GPS 1050 Range 31.6m km Resolution 50
Shepherd-Jordan Active Search Sensor AS8-R1 (50%) (1) GPS 21 Range 8.6m km MCR 771.7k km Resolution 1
Shepherd-Jordan Active Search Sensor AS50-R200 (50%) (1) GPS 4200 Range 50.1m km Resolution 200
Shepherd-Jordan EM Sensor EM1.0-11.0 (50%) (1) Sensitivity 11.0 Detect Sig Strength 1000: 26.2m km
Shepherd-Jordan Thermal Sensor TH1.0-11.0 (50%) (1) Sensitivity 11.0 Detect Sig Strength 1000: 26.2m km
This design is classed as a Commercial Vessel for maintenance purposes
The game throws an error message about a number being to big/small for int32 whenever you try to view the class design, but it can be built via SM. I was unable to make it any larger than shown due to the in32 error messages. However, even if built the ship is immobile because it cannot be refueled, even via SM refuel. It takes about 5 years to load all of the colonists that it can hold.
The cryo chambers seem to be the limiting factor. I could not add more than about 210 000 000 berths worth of cryo chambers before errors started to pop up.
A somewhat smaller version of this vessel might be interesting to attempt. Or if Mr Steve is feeling adventurous raising the int32 limit to int64 might also be interesting.