What was the biggest expansion of the Empire of Man before the open rebellion of Horus Lupercal began (Istvaan V), i.e. what Segmentums / Sectors were controlled by the Empire of Man at that point? And how long did it take the Empire to reclaim those planets?
The largest expansion of humanity took place during M15-M25 the Dark age of Technology. This is when Humanity first discovered warp travel and went out into the galaxy inhabiting and populating planets. However Humanity was still split at this point into the nation states and forces that later on the Emperor would conquer and unify into the imperium of mankind.
Due to the events at the end of that age and then the subsequent age of strife Humanity at that point had lost many planets to a combination of, the war with the iron men and the sudden appearance of and rise in the number of psykers leading to all the destruction that brings.
When the Eldar went and created a new Chaos God not only did the resulting warpstorms cut off worlds and systems from each other and earth, it also resulted in whole planets being destroyed as creatures of chaos found ways into the material realm, largely using the minds and bodies of said psykers.
Therefore by the time the Emperor was ready to start going out and liberating planets much of what was the empire of mankind was no longer salvageable and the planets he "conquered" had all largely been under human control previously, even if there human inhabitants no longer survived.
In regards to how far the Great Crusade had got before the events of the Heresy there is a map
Which shows roughly how far the empire had got in re liberating the human worlds.
However it is important to note that Humanity has never achieved the kind of spread and occupied as many planets as it did during its peak in the dark age of technology and given that many of those worlds where lost and destroyed due to the events during and after that age the empire of mankind will never be as big as humanity was then.
Something to consider is that in the 41st millennium the Empire of mankind has a very definite limit on how far it can spread, that limit is the Astronomican which has a limit of 70,000 light years outside of this distance no vessel can travel safely in the warp, and certainly could not guarantee where it would end up meaning that planets outside of this distance may be populated by Humans, but wont be part of the imperium as Earth will never be able to control them.
So to answer your question. Humanity was at its widest spread in the middle of the dark age of technology before the war with the iron men. it has never matched this since, but, humanity was not at this point an imperium of mankind.
The Imperium of Mankind (the Emperors baby) was at its height just before the heresy began.