The alternate approach is to have them as 'Wonder' units - i.e large-scale feats of mobile engineering. The types of units it would cover would be historical units such as:
Syracusia - Largest transport ship in antiquity
Nemi Ships - Caligula's giant floating palaces
Helepolis - Large movable siege towers
Louchuan - Giant Chinese floating river castles
Warwolf - the largest Trebuchet ever made
Atakebune - Japanese castle Ships
Dardanelles Gun - The Great Turkish Bombard
Schwerer Gustav - Railroad gun, largest-calibre rifled weapon ever used in combat and, the heaviest mobile artillery piece ever built
Hindenburg - Largest Zeppelin
Tsar Bomba - Most powerful nuclear weapon ever created
Of course, the most interesting thing about this list is that while they were all engineering/scientific/mechanical 'Wonders', they were also in many cases either useless or extravagantly wasteful for any real-world purpose:
The
Nemi Ships never left the lake on which they were built. "Floating Palace" is exactly what to call them, because they never actually went anywhere. You might also include here Ptolemy's (in)famous 40-er, the largest Polyreme 'warship' ever built: over 4000 crewmen and solders, but it never left Alexandria harbor!
The
Warwolf, like most of them, was fearsome, but it threw a 300 pound stone weight and just 70 years later almost every Bombard could do the same thing, at higher velocities.
The
Great Bombard took all day to reload and so couldn't be used against anything more mobile than Constantinople. Of course, this was typical of both Bombards and Trebuchets, but only 50 years after the 'Dardanelles Gun' did its dirty work, really mobile artillery with trunnions and trailed carriages made it as obsolete as a traction trebuchet.
Gustav and his Bromdenagian mate Dora were indeed the most colossal Cannon ever built, and like the previous examples, they could only be used against a target suitably Immobile, like Sevastopol. And even then, a single flight of medium bombers could deliver more tonnage of explosives than they could, and with less expenditure of manpower (the crew of Gustav was over 1500 men, not counting the antiaircraft regiment it took to protect it from the enemy's bombers) - and three years after Gustav bombarded Sevastopol, a single B-29 using conventional explosives could deliver twice the weight of Gustav's massive shells, to much greater range and with a crew of just 12 men, a saving of 1488 over
Karl.
Hindenburg -"Oh the Humanity!" - enough said. Although you might also include the "Spruce Goose" or the "Ilya Murometz", each in their day the largest multi-engined fixed-wing aircraft built.
Tsar Bomba, or more accurately the RDS-202
Ivan or
Vanya wasn't actually a 'bomb' at all, it was a test explosive to verify nuclear calculations and explosive configurations for other warhead/bomb designs. Mercifully, nothing derived from that test was ever detonated again, but it remains the most definitive version of a Terror Weapon, which puts it right in the same camp as the WarWolf or Dardanelles Gun - more effective against Enemy Morale than anything else.
Some other candidates for Gargantuan Wonder Construction 'Units' might be:
Nuestra Señora de la Santísima Trinidad, which by 1802 carried 140 guns on 4 full gundecks, making it the most heavily-armed wooden warship ever built. She was also about as nimble as a Hippotamus in High Heels, so got nicknamed "El Ponderoso" by her own crew!
Great Eastern - Isambard Brunel's largest iron ship ever built, so big that it couldn't even be launched without near-disaster and in the event, completely worthless for any commercial or military purpose. It managed, after several tries, to lay a trans-Atlantic telegraph cable.
Maus, the 150-ton Super Tank designed by the Nazis and still unfinished in 1945, which was probably just as well since it was only slightly more mobile than the Brandenburg Gate and so would have made a delightful target for the entire Soviet and US Air Forces.
