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?