The Ecumene is theoretically a large interstellar polity- the largest in its region, if not far beyond. The truth of its existence is rather more complex, however....
The Ecumene was founded as a government (it was not the first in the region) for a cluster of fifteen star systems (all part of what's now called the Ecumenical Main, a group of stars no more than five point six light years apart) and immediately started planning for expansion. The cluster would be divided into small provinces of several stars, each system would be a district... and the cluster itself would be one of around nine hundred subsectors, combining into ever larger divisions up to the fifteen prefecture peak Ecumene. Millenia later, this ridiculous number would be surpassed- but not in the expected way.
Further early history: The Lost Main
Further early history: Edelen/Ticash
Further early history: The Hydric Main
Further early history: The Beamnet
Further early history: The Rockhopper Method
As the Ecumene scraped against the amazing quantity of one thousand star systems- what would later be known as the Centralmost Prefecture (plus a little extra)- it sundered. The six sectors of the Ecumene each gained greater and greater autonomy, and then true independence. When the Ecumene well and truly fell apart, each sector found itself having to face its own struggles largely alone. Most sectors broke into their individual dioceses and subsectors- but one crucial fact was that each polity professed its loyalty to the collapsed Ecumene. The Ecumene worked, after all- even as it was falling apart, it was considered a fair and just force of law and order.
Further middle history: Yclathena/An Infuriating Calendar
Further middle history: Aakernund
The modern Ecumene is maintained one-third by inertia, one-third by wishing to be part of the largest starstate this side of Sagittarius B, and one-third by tax fraud. Immediately before the collapse of the Ecumene, tax collection was redefined to be on a pyramid-hierarchy basis: The Ecumene would levy a tax, each Prefecture would levy its own, and so on down to the lowest level where the administrative division was just real enough that it wouldn't immediately be objected to. This continued even past the collapse of the Ecumene, even into the era where many administrative divisions became irrelevant (some time before the five dominions became official administrative divisions). At the moment the largest somewhat-concrete governing polities are on the scale of single prefectures, and the smallest governing polities are on the scale of Ecumenical viscounties.
There are several large confederal entities that some would consider interstellar nations, each connecting between three sectors and two prefectures, that together fill the entire territory of the Ecumene. These are of varying levels of tangibility, from a weak economic alliance to a semi-functional confederation. Polities that are generally considered, both within and without, as nations range in size between a full hundred-star sector and a small set of domed cities or islands on individual planets (baronies). The Ecumene's current administrative divisions are largely defined from the bottom up- a sector will contain the entirety of all of the cohesive governing polities within it, because it's defined on the basis of those polities. As such, larger administrative divisions can remain relevant for reasons other than tax fraud.
Of course, the Ecumene is merely one of many interstellar polities, albeit one of the largest extant ones. What follows is a set of links describing its subdivisons, current and former polities, and with the species that make them up:
The Trans-Rift Republic/Republic of Trans-Hyperborica
About the Far Stars Prefecture of the Ecumene
Wane Civilizations and other oddities
The Ecumene and its neighbors.
The Ecumene - Planetary, Stellar, and Overstellar Classification
Starways (galaxies), local and otherwise
Miscellaneous monologues and vignettes that fit in the universe of the Ecumene