I don't think there was ever an official statement about it, as GW likes to be obscure and not fix information that is not required to be fixed. But we can calculate it.
Earth is around 26'000 light years from the centre of the milky way:
The black hole known as Sagittarius A* (Sgr A*) is situated in the middle of the Milky Way, just 26,000 light years from Earth, according to NASA. Scientists observing the black hole for four days in April and May of this year using the Keck II Telescope in Hawaii saw the event.
From this article
We know it does not reach the Ghoul Stars:
This region of space lies almost entirely beyond the light of the Astronomican, which is used by Imperial Navigators to guide vessels while travelling in the Warp.
but the T'au Empire is:
The Tau Empire borders the Imperium of Man, and lies within the reach of the Astronomican.
With that information, we can draw a map:
So, we know it reaches 60'000 light years but not 71'000 light years.