Just be that a Leviathan I thought up is about 550,000 KM. It's pretty freaking OP
IDF have no need for such huge ships... Because if enemy drops one of such beasts, then IDF just "replaces" one of the nearest star systems in galaxy to this leviathan with one of IDF's Sphereworlds and then just uses it's weapon systems to destroy it. 100% accuracy range of IDF Sphereworld is about ~50 LY (Light Years), while with targeting guidance it can go up to ~5000 LY...
Then the Leviathan hit's FTT (Faster Than Time, everything is still while you move) travel, begins to drop a fleet of roughly a few hundred thousand ships (50KM+) and replaces them at a lightning rate. Then, if that ain't enough, we can just fire a Scimitar plasma burst (basically, I'm going to put you into a star)... I could also salvo a good 25,000 2KM nuclear (50 teraton) shells in a railgun at you.
That said, the IDF is such like the fleet I thought up. Except we have frickin' nukes. And a lot of them. And a war machine that basically creates insanely huge battles all the time (50KM battleships rolling off a factory every day, 500,000 planets with aforementioned factories, staggered, fleets fully replenished in roughly 2 months).
Ok this is getting ridicules. Faster Then Time? Do you even know the meaning of those words?!
Ya man. Time stands still. You do realise this is sci fi, and I can say the purple green aliens gave me the secret: put bacon in the ftl drives
EDIT: If you want the real truth... If humanity was on the verge of discovering steam power, then the race (or person) that made that ship were working on FTL drives in secret in a project nicknamed 'SHOOTING STAR'. It begins with multiple small ships (roughly the size of corvettes, but for a single person). It takes them a good few years to hit MK 28, the first FTL capable ship. MK 28's blueprints are hard coded into the computer and multiple copies are made, up to MK 32 - the exceptionally infamous Wicked Wanderer (not simply for having the first artificially made biologically accurate intelligence, Wanderer). MK 28 begins to discover a strange materials in tiny quantities in dark matter heavy areas. This material is then used as an additive to the FTL drives of the MK 29, 30, 31, and 32, creating a strange field and level of power in the engines that would be seen, to the pilot, as if everything around him/her, stood still like a photograph, but for onlookers, the ship simply disappeared. FTT travel is not instantaneous - it is more a state of movement than actual travel speed. It does however, being timeless, have the unique property of almost teleportation speeds. FTT is never fully documented, and is not generally possible to reproduce on larger ships. This would've happened roughly by the time of the collapse of the Soviet union. However, the greatest threat known to that race then revealed itself - the tribal, insanely warlike Koltheki. A dark skinned reptilian race not unlike Sangheili, they attacked the station with impunity after discovering it. In the two day assault, every ship up to the MK 28 was destroyed, but the 4 FTT ships survived.
Running back to their homeworlds, pilots then kept in contact with the government to create fleets to beat back the Koltheki. Here began the Weaponization wars, where both races became excellent in their weapons and ship design. The race used nuclear railguns, highly accurate, and very lethal, capable of wrecking fleets from LY away. Koltheki used heavy autocannons and basically are like the Hierarchy in this. They are inaccurate but could kill entire planets with a frigate. Battleship Theory came into play and interstellar battles hit a scale that contained the fury of an entire human war in the stretch of half an hour. The Weaponization wars still exist, but now a greater problem arises. The four original pilots of the FTT capable ships die suddenly, one being the pilot of the MK 32. Wicked Wanderer is insanely lethal once fully baked. In it's prototype form it could face down entire fleets despite being hundredths of the size of enemy ships, using it's interesting weapons package and insane piloting skills (the guy NOW piloting it started off at about 12 years old, he would now be 18). So, as the war continues, a new ship - The Endless Night jumps into orbit of the core world. Oh dear. The Endless Night, a 900KM Leviathan, could fire a missile that could cause a star to go supernovae instantaneously. It uses it's massive weapons package to sweep away the race's fleets, yet before it could fire the missile, Wanderer pulls off an orbital strike - throwing a nuclear shell directly into the bridge, tearing apart internal control systems, and then ultimately perishing.
As for the 550,000 KM Leviathan (The Guide)... let's just say that bending starmatter to the will of a forge is easy once you find the right technology.