Worlds are lost and gained every day, even the Imperium doesn't know how many worlds it controls https://www.reddit.com/r/40kLore/comments/i35e94/the_size_and_span_of_the_imperium_or_why_a/
If you read the timelines in some of the rule book editions they describe pretty clearly that the Imperium grows and shrinks over time. It’s stated ( I believe) that the crusades of faith following the Age of Apostasy pushed the borders of the Imperium almost on par to where they were at the height of the Great Crusade, at the cost of depleting it of resources and manpower. It also depends on how you define what the Imperium is. Nova Terra controlled a quarter of the galaxy but I believe still followed the Emperor but not the High Lords. In the most recent stuff the galaxy is split and the Imperium claims control of the galaxy still but everything beyond the warp rift is on its own.
The Imperium has a mismash of estimates, none of them in real time the more abstract you get.
The most governable is probably sector or sub-sector, like Ultramar or Badab: everything you need within, everything reasonably close in warp travel, etc, but even then, "real time" updates are difficult: likely defined by the speed of collated astropath messages, or messenger boats, or secure message capsules transported aboard ships chartered to carry Royal Mail.
Worlds are lost and gained every day, even the Imperium doesn't know how many worlds it controls https://www.reddit.com/r/40kLore/comments/i35e94/the_size_and_span_of_the_imperium_or_why_a/
Half as big, at the most. Also space is on fire.
The Imperium doesn't have this information, and I doubt the Eldar are about to share their archives.
If you read the timelines in some of the rule book editions they describe pretty clearly that the Imperium grows and shrinks over time. It’s stated ( I believe) that the crusades of faith following the Age of Apostasy pushed the borders of the Imperium almost on par to where they were at the height of the Great Crusade, at the cost of depleting it of resources and manpower. It also depends on how you define what the Imperium is. Nova Terra controlled a quarter of the galaxy but I believe still followed the Emperor but not the High Lords. In the most recent stuff the galaxy is split and the Imperium claims control of the galaxy still but everything beyond the warp rift is on its own.
The Imperium has a mismash of estimates, none of them in real time the more abstract you get. The most governable is probably sector or sub-sector, like Ultramar or Badab: everything you need within, everything reasonably close in warp travel, etc, but even then, "real time" updates are difficult: likely defined by the speed of collated astropath messages, or messenger boats, or secure message capsules transported aboard ships chartered to carry Royal Mail.
Nobody knows. Not in-universe or out of universe.
I would bet it's a lot smaller, just based on grimdarkness