I think it’s more vast than you think, given how many different areas of land it includes:
Lord Jonos drained the last of his ale and tossed the horn aside. “What of the lands and castles we were promised?”
“What lands were these?”
“The east bank of the Widow’s Wash, from Crossbow Ridge to Rutting Meadow, and all the islands in the stream. Grindcorn Mill and Lord’s Mill, the ruins of Muddy Hall, the Ravishment, Battle Valley, Oldforge, the villages of Buckle, Blackbuckle, Cairns, and Claypool, and the market town at Mudgrave. Waspwood, Lorgen’s Wood, Greenhill, and Barba’s Teats. Missy’s Teats, the Blackwoods call them, but they were Barba’s first. Honeytree and all the hives. Here, I’ve marked them out if my lord would like a look.” He rooted about on a table and produced a parchment map.
Jaime took it with his good hand, but he had to use the gold to open it and hold it flat. “This is a deal of land,” he observed. “You will be increasing your domains by a quarter.”
Bracken’s mouth set stubbornly. “All these lands belonged to Stone Hedge once. The Blackwoods stole them from us.”
“What about this village here, between the Teats?” Jaime tapped the map with a gilded knuckle.
“Pennytree. That was ours once too, but it’s been a royal fief for a hundred years. Leave that out. We ask only for the lands stolen by the Blackwoods. Your lord father promised to restore them to us if we would subdue Lord Tytos for him.”
The entire bank of a river, two ruined strongholds and one ruined town, a valley, four villages (maybe five depending on whether Honeytree is a village, a town, or a stronghold), a market town, two woods, three hills…and all of this is just the disputed lands between House Bracken and House Blackwood that would represent a fourth of House Bracken’s current holdings.
To me, this suggests that House Blackwood probably holds most of the lands between the Red Fork and the Blue Fork (there’s not a lot of other Houses from that region – notably Fairmarket and Oldstones don’t seem to have houses associated with them) and a fair bit of land on the southern bank of the Red Fork, given Lord Bracken’s complaints. For their part, I think House Bracken controls most or all of the hill country east of Pinkmaiden and west of Harrenhal and from the south bank of the Red Fork to however far north Harrenhal’s demense runs.

In other words, between them the Bracken and Blackwoods likely control most of the central Riverlands, making them extremely difficult to control, especially if they temporarily band together against an outside intervention.