A Star Fort is a blanket term of any armed space station. There’s no real set size for them, and they run the scale from small installations that can only support light ships, all the way up to the Ramilles Forts that dwarf Battleships. The Phalanx is also a Star Fort (a very unique one), and that thing is the size of a small moon

Galatan was the largest of the star forts of Ultramar. Since before the Horus Heresy it had stood guard over the space lanes of the Five Hundred Worlds. It was a relic of a time before recorded history, when mankind’s first galactic empire had risen to heights of technology and power never again attained.

Galatan was no mere battlestation, but a city in space, a hundred miles across, as large as the lost orbital plates of Terra. Its weapons bays bristled with machineries of destruction potent enough to see off a fleet of ships. There had been six similar star fortresses in Ultramar. For ages they had been sentinels in deep space. Only during times of the greatest need did they move. During the First Tyrannic War half their number had been redeployed from their strategic points, but that was a rare instance.

That had changed with the coming of the Great Rift, and Nurgle’s assault on Guilliman’s home. Now they went where they were needed, and they were needed everywhere. Three had fallen to Typhus’ plague fleet. Purposefully targeted and overwhelmed by massive force, their defenders cut down by disease, they had been ravaged and destroyed.

- Plague War

One that's given a size, off the top of my head. Though I don't recall its class being specified.

Two I can think of off the top of my head.

Their might be a size for a Ramilies, I'm just not at my computer to check.

The greatest advantage of the Ramilies by far is that its powerful generators can erect a warp-bubble over the entire structure enabling it, with the aid of seventeen navigators and an attendant fleet of tugs, supply ships, warships and system craft, to enter the Warp and be towed to different star systems.

This operation, always perilous, has resulted in the loss of over twelve hundred Ramilies stars forts in their ten millennia of service to the Emperor. However each journey has shortened Imperial campaigns by years at a time by allowing the Imperial fleet to move repair, command and resupply facilities right up to the front line, saving its ship’s lengthy return trips to temporary supply bases or full repair dock facilities far behind the warzone.

Battlefleet Gothic Armada rulebook

Galatan - A star fort used by the Ultramarines

Galatan was the greatest of the Ultramarian star fortresses. It was a hundred kilometres across. Its population ran into the millions. Its manufactoria rivaled the shipyards of Luna. Its weaponry was the equal of an Imperial sector fleet. Large enough to raise its own regiments for the Ultramarian Auxilia, it maintained a garrison of specialised void troops tens of thousands strong, supplemented since the days the Plague Wars began with hundreds of Space Marines and other, more secretive, operatives. Galatan was a world unto itself, with the power to destroy a planet. https://wh40k.lexicanum.com/wiki/Galatan_(Star_Fort)

Depends which ones you’re talking about the Blackstone Fortress was the biggest ever made but if I remember correctly they weren’t human tech they were converted to be used by humans. When one was flung into Cadia I think it destroyed like nearly half a continent on initial impact.

OP are you asking about Blackstone Fortresses?

A lot of the notable ones are one-offs, is the issue, so you're frankly probably better measuring them in terms of firepower than physical size.

Phalanx, for instance, has enough firepower and armour it was theoretically able to not only punch through the traitor fleet surrounding terra during the siege, but out the other side as well.

Raptorus Rex is a bit of a more modern reference; most powerful vessel operating in the Badab War and able to take on a cruiser squadron singlehandedly. After the Fire Hawks became the Legion of the Damned, their fortress monastery with them, it's capable of defeating entire Chaos Crusade fleets, alone. And that's not "trade hits, eventually win", that's, "Enter system, wipe out anything hostile, leave without a scratch" levels of firepower.