Would you say that the Hornwoods are great lords of the North on the same scale as the Manderlys or the Boltons? Corollary: how many vassals might the Hornwoods or Boltons have, and what’s the relative size of their lands?

The Hornwoods are significant players in Northern politics, but they don’t seem to be as big as the Manderlys and Boltons, not least because they’re not able to stop the Boltons and Manderlys from using them as a battleground in a contest between those two houses. 

The Boltons have enough land and vassals to raise 3,000-4,000 men, although that’s a lot less than they used to have before the Starks raised up the Karstarks and the Manderlys and rewarded the Umbers, all from Bolton lands.