«At the edge of the known galaxy, where Palpatine’s holo had shown only the white stars of the Unknown Regions, an entirely new area had been colored in. A huge new area. (...) Luke gazed at the holo, listening to the distant hum of the spiral slideway, the terrible implications of those gently glowing lights tumbling over each other in his mind. There had to be the equivalent of two hundred fifty sectors there? Nearly thirty times the Empire’s current size.»
Vision of the Future, Timothy Zahn, 1998
Luke considering the size of the Empire of the Hand's territory.
250 sectors is insane! That's the equivalent of a quarter of the Galactic Empire's territory at its peak. It also translates in the whole northern part of the Unknown Regions down to the borders of the Ascendancy.
It also means that, had he been succesfull in overthrowing the New Republic, Thrawn would have had control over near 3/4 of the Galaxy.
It would have been one of the largest empires in the history of Star Wars... but it also would have been very short lived. Thrawn's never been much of a politician even in Legends. His leadership is respect-based and I'm afraid the inevitable internal power struggle which would flare up among imperials would have Thrawn toppled. His fate would not be much different from Pellaeon's... not to mention Thrawn also had a score to settle with the Ascendancy.