We don’t know exactly. Given that it’s said to be one of the richest fiefs in all of Westeros, it must be extensive.
Incidentally, this is yet another reason why the Riverlands having only 20,000 swords is iffy. If Harrenhal is that significant on a continental scale, it’s got to have more men than the Freys if somewhat less than the Hightowers b/c it doesn’t have a city associated with it. So why did Shella Whent yield Harrenhal to Tywin?
Yeah that’s been my issue with GRRM’s handling of the Riverlands as well hopefully he’ll flesh it out more later on. The Lords of the Riverlands seemed more interested in preserving their holdings from being torched so the troop numbers raised are probably way lower than they would if the whole region wasn’t being turned into a slaughter house.
As for why Lady Whent yielded Harrenhal, I had assumed that she wasn’t all quite there to be honest. With her family all dead and have been left alone to herself in Harrenhal for well over a decade her mind has probably deteriorated As Cat mentioned in her V POV. If that’s the case then she maybe didn’t even register that there was a threat at all till Tywin showed up at her doorstep with his army, and without having called up her levies she’s left with only probably a small garrison to defend a castle the size of a city.
So if we’re going by that troop estimate would you agree that Harrenhal could possibly field around 10k?
See, that still doesn’t make sense to me. Because Twin marches across the Riverlands at speed, capturing Pinkmaiden Castle, Atranta/Wayfarer’s Rest, Raventree Hall, Darry, Harrenhal, without taking a scratch. Given that he didn’t have the time to starve them out, he had to have been assaulting them. If the Riverlords were keeping their men home, those assaults should have been high-casualty affairs like we see later in the series, given the defensive advantage that castles offer. Yes, there are some exceptions, but not enough to explain why Tywin’s army is completely untouched.
I would say that Harrenhal probably has 5-6k at the least, 8k at the most, given that a lot of revenue probably gets eaten up on upkeep.