The largest purely flying vehicle archaeologically verified to have been constructed within the Ecumene's region of space was Tristellar Admiral-General Ulvatewe's Floating Fortress, a self-propelled military installation a dozen kilometres wide. Several hundred years after the Ecumene broke apart, the disparate nations that formed it were orderly enough to start genuinely expanding again. First of these was sector 004's Republic of Clearsky (the name is a long, confusing, and contradictory story); which once it was established began gradually expanding the Ecumene past the nine hundred stars it had held onto. One of the larger states to join the Ecumene was the Interstellar Federacy, a seventeen-star state that saw the Ecumene as an opportunity to save itself from the brink of political collapse. Though officially the Federacy joined the Ecumene on 318.2911, and official Federacy outlets in all seventeen systems were spreading the news throughout 2912 and 2913, the Federacy's components were more divided on the matter.
In particular, a segment of the Federacy that had gained de facto independence in 2908 was understandably rather opposed to it. This segment was quickly taken by jingoistic rhetoric, such that Admiral Of The Stellar And Seaborne Navies Amany Ulvatewe was able to become acting leader. Ulvatewe quickly ramped up military production in the two star systems he had jurisdiction over, and began constructing an airborne fortress-capital-palace for himself. Planetside antigravitic propulsion was common in the Federacy, so Ulvatewe's capital could be enormous.
It was largely completed by the time what passed for an Ecumenical government in sectors 004 and 042 took notice of Ulvatewe's dictatorship in 2927- only the systems that would allow it to reach space were incomplete. When emissaries from Clearsky alerted Ulvatewe that his continual crackdowns on anti-government action would not go unnoticed for long, Ulvatewe prepared for what he considered an inevitable assault from orbit- which he decided he could defend against by filling the atmosphere with laser-deflecting smog. When ships from Clearsky arrived, he had an ambush prepared- and of the few ships that made it out, only one was able to close to a distance where it could attack the fortress. The fortress quickly shot the already-damaged ship out of the sky... but after descending through the smog, it slammed into the desert with a glancing blow, sending kilotons of soil at the fortress.
A key hit disabled its central forward gravitic deflector, and the other deflectors could not handle the strain. For an instant, all eight deflectors shut down... then the rear ones flickered back on, slamming the bridge of the fortress into the ground. Ulvatewe was killed on impact, and by 2936 his smog had settled over his gutted fortress. For millenia this was considered a tale made up to convince foreign peoples to join the Ecumene, but in 4503 a devoted survey uncovered much of the crumpled remains of the fortress. Several sections of the fortress have been restored to a condition reflective of their ancient status, but much of its vast bulk is being maintained in the condition it was in when it was found in 4503.
The Fist of Artay is a possibly-mythical human spacecraft claimed to have been the largest spacecraft ever built. It was to be the ultimate expression of the power of humanity, specifically the Seventeenth Galactic Empire of the early 1000s, and while purported sizes vary all agree that it was massive enough to be in hydrostatic equilibrium. While its statistics are all very impressive, and almost expected- kilogee acceleration, megacities to house the crew, millions of guns, and several new types of FTL invented to move the thing- the true oddity is that despite such evidence for its existence, no trace of the Fist of Artay itself has ever been found. Accounts from both the 17th Empire and its rivals attest to its existence, though much of what remains is distorted- likely leading to the many conflicting reports of its size and details. News of the Fist of Artay has spread everywhere reachable by the Ecumene, and then some- in fact, when the Ecumene first learned of the Fist of Artay (through the usual channels- sailor tales and hearsay) in 1826 the news came from the side of the Ecumene facing away from Artay.
Still, very little directly connected to the Fist of Artay exists- crew logs have made their way along the spacelanes, as have thousands of craft and people rumored to have been aboard at some point or another; but no actual substance of the ship itself besides some chunks of metal claimed to have been forged in the ship's vast fabricators has been found. This is not to say there is no physical evidence for its construction- dedicated construction bays, repair yards, supply ships, and escort craft have been discovered. However, every last thing claiming to be a piece of the Fist of Artay itself has turned out to be a fake. Some frauds have been very convincing- a shuttlebay containing authentic 17th Empire spacecraft stood for centuries before it was discovered that the shuttlebay was made of ten shuttlebays from smaller 17th Empire ships welded together- but even the best end up false.
Many theories have been put forward as to why the Fist of Artay has never been found. A plurality proclaim that it was lost in some horrifying cataclysm of some type or another, often with the caveat that it was wiped from existence. A slightly smaller amount state that, as the Seventeenth Galactic Empire was renowned for long-lasting giant machines (just take the Emissary of Siellau, which as of 8722 can still theoretically be talked to as it burns its way out of the galaxy on its fakedrives), the Fist of Artay must still be out there somewhere- perhaps pretending that the Seventeenth Galactic Empire is still extant. Sources from the area that the Seventeenth Galactic Empire claimed (now somewhat disparate due to the time that's passed since, with stellar drift meaning one star is even easily accessible from the Ecumene) say that it is not lost- but it is not found either.
FTL technology has many different interpretations, from alternate world-volumes to interstellar railroads to just plain ignoring physics. However, few methods yet available have breached the six-hours-per-light-year barrier, even unsafely. All of these are ancient artifacts or highly selective- except, technically, one. A project in the Manakay system in 8648 sought to develop such a drive by the opposite path as normal- rather than improving or building upon FTL principles already developed, why not aim for something completely beyond current FTL technology? Their aim was a craft that could travel ten light-years per day. After countless tests and a few years, all the scientists were so thoroughly tired of the maddening intricacies of the project that they bundled into the test starcraft and activated the drive.
They had constant, debilitating nausea for a full week, as did the ship's computer. And all of the parts of the ship that were aware enough to register nausea. But when it was finally, mercifully over they found themselves on the very edge of the "mainland" Ecumene, orbiting a planet that had chosen to join so recently that the news hadn't reached Manakay yet. They had travelled six hundred seventy light-years in six days, perhaps a thirty-third of the time that would be needed by even the fastest other drive- and, of course, outpacing the ansible-based news of their departure by over two years.
They were, of course, deeply reluctant to attempt to activate the drive again. Apart from the awful nausea, they were unsure it would even return them to Manakay. However, planetary authorities were deeply interested in the possibility of such a drive... but their attempts at activating it didn't work. Even the scientists were unable to get the drive working again- all of the prerequisite steps worked fine, but the drive never activated. As of 8724, the compilation of their more recent work Why The Nausea Dimension Is Forever Barred To Us is still being written.
The pirate cruiser Eldam Anneka has won awards for having perhaps the most overly dense transit system for its size. A ship scarcely 160 metres long, let alone one so flat, should only require a horizontal elevator at the most- but Eldam Anneka features vehicles fit for a ship many times its size, or a transcontinental system for a low-population world. Two vehicles circulate at all times, while another two remain in reserve- for each vehicle can also serve as the habitation section of an escape pod, complete with a kitchen and a toilet. Three of five decks feature tracks and stations for the vehicles, complete with vertical stations built into the fore bridge, middle command, and rear engineering rooms. Even in pirate annals, Eldam Anneka is well-reknowned for its crew being so good that the extreme ease of taking the ship if it was boarded is an immaterial risk.