I did a million ton dreadnaught, back in 1982 IIRC; I do not remember the specifics, other than I put ten spinal mounts on it (I KNOW that's breaking the rules). I basically said they would all share the same to hit and damage rolls. I don't recall there being a huge advantage.
More recently, I realized HGS 2.0 did not enforce the Mega dton limit, so created a small moon...

o:
Ship: DS1
Class: Death Star
Type: Battle Station
Architect: Ballard Designs
Tech Level: 15
USP
BX-Z5424JZ-F99949-99T99-Y TCr 9,604.211 10,000 MTons
Bat Bear [see below] Crew: 67647768 TL: 15
Cargo: 245,808 Passengers: 90000 Fuel: 5,800,010,000 EP: 400,000,000 Agility: 1 Marines: 1,000,000 Drop Capsules: 50,000 (plus 200,000 Ready) Fitted as Flagship: Accommodation for Admiral and ten staff with Flag Bridge and Model/8fib Flag Computer
Craft: 1,000 x 50T FHes, 5,000 x 50T Cutters, 20 x 400T Patrol Cruisers, 40 x 400T Fleet Couriers, 5,000 x 8T Astrin G-Carriers, 4 x 60000T Azhanti High Lightnings, 40 50T Launch Tubes, 2 400T Launch Tubes
Fuel Treatment:
Fuel Scoops and On Board Fuel Purification
Backups: 1 x 1G Maneuver Drive 1 x Jump 1 Drive 1 x Factor 1 Power Plant 4 x Model/9fib Computers 1 x Bridge 3 x Factor 9 Nuclear Dampers 3 x Factor 9 Meson Screens 1 x Factor 4 Black Globe
Architects Fee: MCr 96,037,368.923 Cost in Quantity: MCr 7,683,463,513.840
COMMENTS
The Death Star is a large military city that is capable of jump-4.
At 10 billion dtons, it is a massive structure, a sphere 62 km in diameter.
It has a staggering 800,000 Factor-9 beam laser batteries, 500,000
100-ton missile bays, and 100,000 100-ton meson bays.
It carries 1,000,000 marines who can be landed in 4 waves of the station's
5,000 cutters, and supported by 1,000 heavy fighters.
It has a crew of 45,000,000, and carries up to a year's supplies for them.
Life support alone runs 2.34 billion annually.
It's cost was 9 Quadrillion, 6 Hundred Trillion credits (9.6x10^15 Cr).