On the Game of Thrones website it never says it but does anyone know how much land Harrenhal controls?
Major note: This information is heavily based on the books in ASOIAF series. However, the information should translate to the show as the events surrounding Lord Baelish's appoints are the same
Based on current events Harrenhal controls all the land associated with the Riverlands. This occurs when Petyr Baelish is awarded the castle of Harrenhal to make his family seat. This was awarded to him by King Joffrey after the Battle of Blackwater for forging the alliance of Lannisters and Tyrells. In addition to being the Lord of Harrenhal, he is also named Lord Paramount of the Trident. which was typically held by the Tully family and Riverrun.
Ιt is the wish of the King's Grace that his loyal councillor Petyr Baelish be rewarded for faithful service to crown and realm. Be it known that Lord Baelish is granted the castle of Harrenhal with all its attendant lands and incomes, there to make his seat and rule henceforth as Lord Paramount of the Trident. Petyr Baelish and his sons and grandsons shall hold and enjoy these honors until the end of time, and all the lords of the Trident shall do him homage as their rightful liege. - A Clash of Kings, Chapter 65
To get an idea of the size of the area and notable towns that are a part of the Riverlands, have a look at this map. The Riverlands are those inside the black line.